causes of infectious disease
For as long as there have been humans, there have been infectious diseases. Also known as communicable diseases, infectious diseases (in FEK shus) are caused by organisms or viruses that enter and multiply within the human body. Most disease-causing organisms and viruses are so small that they can be seen only through a microscope. Organisms this small are called microorganisms (my kroh AWR guh niz ums).
Not all microorganisms that enter and live in your body cause disease. In fact, many are present in your body all the time. Billions of microorganisms live in your mouth, on your skin, and in your digestive tract.
Microorganisms that cause disease are called pathogens (PATH uh junz). Pathogens do not belong in your body. Pathogens can cause an infectious disease when they enter your body and multiply.
There are many kinds of pathogens. The most common are bacteria, viruses, fungi and protozoans (pro tuh ZOH unz).
bacteria
Bacteria (bak TEER ee uh) are simple, single-celled microorganisms. Bacteria live in air, soil, food, and in and on the bodies of plants and animals, including you. Most bacteria are not pathogens.
Some bacteria injure cells by giving off poisons called toxins (TAHK sinz). Certain bacteria that grow on food, for example, give off toxins that can cause food poisoning. A type of bacteria found in soil produces a toxin that causes tetanus (TET n us). These bacteria can also grow inside deep wounds. In the body, tetanus toxin damages the nervous system, causing uncontrollable muscle contractions, paralysis, and even death.
viruses
The smallest pathogens are viruses. They are about 100 times smaller than most bacteria. Unlike most bacteria, a virus can multiply only after entering a living cell. The virus then takes over the cell's reproductive mechanisms, resulting in cell damage or death.
Some viruses, such as those that cause the common cold, invade the cells of the respiratory tract. Other viruses invade other regions of the body. The virus that causes chickenpox, for example, invades skin cells.
fungi
Organisms such as yeasts, molds, and mushrooms are known as fungi (FUN jy). Fungi grow best in warm, dark, moist areas. Two examples of disease caused by fungi are athlete's foot and ringworm, a skin infection that forms a reddish circle on the skin.
protozoans
Single-celled organisms that are much larger and more complex than bacteria are known as protozoans (proh tuh ZOH unz). Protozoans have the ability to move through fluids in search of food. Malaria, a disease that is common in tropical areas, is caused by a protozoan that infects red blood cells, causing weakness and nausea. Amebic dysentery is caused by a different protozoan. Dysentery is characterized by stomach pain and diarrhea.
other pathogens
Some infectious diseases are caused by animals such as mites, lice, and certain worms. For example, the trichina worm can live in the muscle tissue of some animals, such as pigs. If the meat of an infected animal is not thoroughly cooked, a person who eats the meat can become infected.
how pathogens spread
How does a person come in contact with pathogens? Pathogens can spread through contact with an infected person; an infected animal; contaminated objects; or contaminated food, soil, or water. The pathogens can then enter the body through breaks in the skin or through the moist linings of the eyes, ears, nose, mouth, or other openings.
infected people
Many infectious diseases are spread through some form of contact with a person who has the disease. The contact may be direct physical contact, such as shaking hands or kissing. If you kiss someone with a cold sore, for example, the cold sore viruses could enter your body. Sexually transmitted infections are transmitted through direct physical contact.
Infectious diseases can also spread through indirect contact. For example, if an infected person coughs or sneezes, you can inhale the pathogens in tiny droplets of moisture in the air. Influenza, measles, mumps, and chickenpox can spread by droplet inhalation. Contact with an infected person's blood, such as when needles are shared to inject illegal drugs, is another form of indirect contact that can spread disease.
infected animals
Some infectious diseases are transmitted to humans through the bites of animals. For example, rabies, a deadly disease of the nervous system, can be transmitted by bites from infected dogs, bats, or raccoons. Malaria is spread through mosquito bites. Lyme disease and Rocky Mountain spotted fever are spread through tick bites.
contaminated objects
Some pathogens can survive for a period of time outside a person's body. These pathogens can be spread from person to person on objects such as doorknobs, eating utensils, towels, and needles used for body piercings and tattoos. If you drink from a cup used by an infected person, you can become infected as well. If you touch a desktop or money that has been sneezed or coughed on, or contaminated in some other way, you can become infected when you touch your eyes, your mouth, or your food. This is why it is always a good idea to wash your hands often, especially before eating.
contaminated food, soil, or water
Some pathogens are naturally present in food and soil. One common type of food poisoning is caused by Salmonella bacteria, which can live in poultry and eggs. Another type of food poisoning is caused by E. coli bacteria, which can live in beef. It is important to cook foods thoroughly to kill these bacteria. It is also important to refrigerate food promptly to prevent the growth of harmful bacteria.
Other bacteria can live in foods that have been improperly canned. These bacteria cause botulism (BAHCH uh liz im), a very serious and often deadly type of food poisoning. The bacteria that cause tetanus are naturally present in soil. These bacteria can enter your body through cuts on your skin.
Sometimes, water and food become contaminated with pathogens from infected people. Drinking water contaminated by sewage is a common source of disease in many areas of the world. Cholera, for example, is a bacterial disease of the digestive system that causes severe diarrhea. Cholera outbreaks can occur after floods and earthquakes, when water and sanitation systems are disrupted.
physical and chemical defenses
If pathogens are everywhere, why aren't you sick all the time? When you do get sick, what keeps the pathogens from multiplying until they take over your body? The answer to these questions is that your body has a number of defenses against infection. Your body's first line of defense against infectious disease includes both physical and chemical defenses that prevent pathogens from entering your body.
skin
Your skin serves as both a physical and a chemical barrier against pathogens. The surface cells are hard and have no gaps between them. Sweat acts as a chemical barrier because it contains acids that kill many bacteria. Finally, old skin cells are shed constantly, and the pathogens on these cells are shed, too. Microorganisms usually cannot get through your skin unless you have a cut, scrape, burn, or other injury.
mucous membranes
The openings into your body, such as your mouth, eyes, and nose, are covered by protective linings called mucous membranes (MYOO kus). Mucous membranes secrete a liquid called mucus. The mucus traps many pathogens and washes them away. Mucus also contains chemicals that attack pathogens.
cilia
Some of your body's mucous membranes are lined with tiny hair-like structures called cilia. Your air passages, for example, are lined with cilia. Together, cilia and mucus help trap and remove pathogens. When you inhale, dust and pathogens get trapped in the mucus of your air passages. The cilia beat rhythmically, moving the mucus up your windpipe toward your mouth and nose. When you cough, sneeze, or blow your nose, the pathogens are removed along with the mucus.
saliva and tears
Your saliva and tears can trap pathogens and wash them away. Like mucus, saliva and tears also contain chemicals that attack pathogens.
digestive system
Chemicals in your digestive system, including acids in your stomach, kill many pathogens. In addition, the normal motions of the digestive system not only move food through your system but also move pathogens out. Finally, bacteria that normally live in your digestive system produce substances that can harm or kill invading bacteria.
inflammation
If pathogens are able to get past the physical and chemical defenses and begin to injure cells, your body is ready with its second line of defense�inflammation (in fluh MAY shun). Inflammation is your body's general response to all kinds of injury, from cuts and scrapes to internal damage. Inflammation fights infection and promotes the healing process.
phagocytes
Within seconds after your body is injured, the damaged cells release chemicals that cause blood vessels in the injured area to enlarge. Blood, other fluids, and white blood cells called phagocytes (FAG uh syts) leak out of the enlarged vessels. The phagocytes engulf and destroy pathogens. Meanwhile, the infected area becomes red, swollen, and sore�in other words, inflamed.
healing
Phagocytes also give off substances that cause healing to begin. The fluids, phagocytes, and dead cells that accumulate at the injury site often result in the formation of a thick, white liquid called pus. Eventually, the inflammation process heals the damage, and the inflammation subsides.
immune system
Your body's third and most sophisticated line of defense against pathogens is your immune system. The immune system (ih MYOON) fights disease by producing a separate set of weapons for each kind of pathogen it encounters
the immune response
When a pathogen enters your body for the first time, it often causes disease. If your immune system is working, why does this happen? The explanation is that your immune system must build up its arsenal of weapons against the pathogen. This process takes time, during which the pathogen multiplies in your body and causes disease. Once the immune system's arsenal is built up, however, the immune system kills the pathogen, and your body gradually recovers. White blood cells called lymphocytes (LIM fuh syts) carry out most of the immune system's functions.
What happens if a pathogen that has previously attacked your body enters your body again? This time, your immune system will quickly recognize the pathogen and launch an immediate attack. When this happens, you are said to be immune to the disease. Immunity (ih MYOON ih tee) is your body's ability to destroy pathogens that it has previously encountered before the pathogens are able to cause disease.
t cells
There are two types of lymphocytes�T lymphocytes, or T cells, and B lymphocytes, or B cells. T cells perform several functions.
Killer T cells destroy any body cell that has been infected by a pathogen.
Helper T cells produce chemicals that stimulate other T cells and B cells to fight off infection.
Suppressor T cells produce chemicals that "turn off" other immune system cells when an infection has been brought under control.
T cells also help your immune system "remember" pathogens. This memory capacity, along with the memory capacity of B cells, is what causes you to develop immunity to a previously encountered pathogen.
b cells
The B lymphocytes, or B cells, produce antibodies. Antibodies (AN tih bahd eez) are proteins that attach to the surface of pathogens or to the toxins produced by pathogens. This binding action keeps the pathogen or toxin from harming the body. Each type of B cell produces antibodies that attack a specific pathogen or toxin.
Once an infection is overcome, your B cells stop producing antibodies, but they do not "forget" how to produce them. Those B cells continue to circulate in your body for years. They are ready to produce antibodies quickly if the same pathogen reenters your body. This memory capacity of B cells explains why you develop immunity to some diseases you've already had.
T cells and B cells work together when fighting pathogens
lymphatic system
Much of your immune system is contained within your lymphatic system. The lymphatic system (lim FAT ik) is a network of vessels that collects fluid from your tissues and returns it to the bloodstream. The fluid flowing through the lymphatic system is called lymph (limf).
The lymphatic vessels have hundreds of small stations, called lymph nodes. Each lymph node acts as a sort of filter. Phagocytes and lymphocytes are present in the lymph nodes and attack pathogens as they pass through.
passive and active immunity
There are two types of immunity�passive and active. Both types are important in protecting your body against infections.
passive immunity
People can develop immunity if they are given antibodies against a pathogen. Immunity acquired by receiving antibodies from a source other than one's own immune system is called passive immunity. This type of immunity is temporary, not lifelong. It occurs naturally in babies, who receive antibodies from their mothers before birth. After birth, antibodies also pass to an infant in the mother's breast milk. These antibodies protect newborns before their own immune systems have fully developed.
Passive immunity also can be artificially acquired. If you were bitten by a dog with rabies, a doctor would give you injections of rabies antibodies to prevent the disease. Eventually these antibodies would disappear from your body.
active immunity
Immunity that your own immune system creates is called active immunity. Active immunity results from either having a disease or from receiving a vaccine.
When you were a baby, you may have received injections to protect you from some common childhood infectious diseases, such as measles, mumps, and rubella. These injections, which caused you to become immune to the disease, are called immunizations (im yuh nih ZAY shunz), or vaccinations. The substance that is injected is called a vaccine (vak SEEN). Vaccines contain small amounts of dead or modified pathogens or their toxins.
A vaccine causes your immune system to produce antibodies against the pathogen, as if you had actually been infected. You develop immunity without having to experience the disease. After a few years, you may receive a booster dose of some vaccines to "remind" your immune system to maintain your immunity.
Many people need additional immunizations because of the work they do, the places to which they travel, or their exposure to an unusual pathogen. Other people may need to be immunized because of risk factors such as age or poor health. Influenza vaccines, for example, are given to elderly people to protect them from the flu.
bacterial diseases
Despite all your body's defenses, occasionally pathogens are able to enter your body and cause disease. In most cases, your immune system fights the infection and you gradually recover. There are thousands of infectious diseases, and over 40 kinds commonly occur in the United States. Four infectious diseases caused by bacteria are strep throat, Lyme disease, meningitis, and tuberculosis.
strep throat
A bacterial disease that is common among teens is strep throat. "Strep" is short for Streptococcus, the bacterium that causes the disease. Strep bacteria, which are usually found in the nose and throat, can be spread by contact with mucus from an infected person. Symptoms include sore throat, swollen lymph nodes on the sides of the neck, headache, and fever. A fever is a body temperature above 98.6�F and usually indicates that your body is fighting an infection. A doctor can diagnose strep throat by swabbing the back of your throat and identifying the bacteria in the sample.
lyme disease
Humans can become infected with the bacteria that cause Lyme disease when they are bitten by an infected tick. Symptoms of Lyme disease include a red rash at the site of the tick bite, fever, chills, and body aches. The best way to protect yourself from Lyme disease is by avoiding tick bites. In wooded areas, wear long-sleeved shirts and long pants, and tuck your pants into your socks.
bacterial meningitis
An infection of the fluid in the spinal cord and the fluid that surrounds the brain is called meningitis. Symptoms of meningitis include high fever, headache, vomiting, and a stiff neck. Anyone who experiences these symptoms should seek medical attention immediately. There are two types of meningitis�one is caused by bacteria, the other by a virus. Bacterial meningitis tends to be more serious than the viral kind. Early treatment of bacterial meningitis is critical to prevent serious illness and death.
tuberculosis
A highly contagious bacterial infection of the lungs is tuberculosis (too bur kyuh LOH sis), or TB. It is transmitted when droplets from an infected person's cough or sneeze are inhaled. Symptoms, which include fatigue, weight loss, a mild fever, and a constant cough, may not show up for many years after the initial infection. One-third of the world's population is infected with TB, and nearly two million people die of TB each year.
treating bacterial diseases
If your doctor determines that you have a bacterial disease, he or she may treat your infection with an antibiotic (an tih by AHT ik), a drug that inhibits or kills bacteria. An antibiotic is a prescription medicine�a medicine that is available only with a written order from a qualified healthcare professional.
Using antibiotics exactly as they are prescribed is very important to prevent bacteria from developing resistance to the medicine. Antibiotic resistance can result if you don't finish your prescription and some of the bacterial pathogens in your body survive. For instance, some TB bacteria have developed antibiotic resistance, making them very difficult to treat.
viral diseases
Some diseases are caused by viruses that enter the body and multiply. Viral diseases include the common cold, influenza, pneumonia, and hepatitis.
the common cold
The common cold is really a group of symptoms that is caused by a number of different viruses. One or two days after exposure to a cold virus, people develop sneezing, sore throats, runny noses, coughing, chest congestion, fever, headaches, and muscle aches. Most colds last three to seven days. Colds spread when a person touches a contaminated object or inhales droplets from a sneeze or a cough. Unfortunately, there is no cure for the common cold.
influenza
The flu, or influenza, is a common viral infection of the upper respiratory system. Like the common cold, influenza is spread by airborne droplets and contact with contaminated objects. High fever, sore throat, headache, and a cough are typical symptoms of the flu.
Influenza viruses can cause more serious illness than cold viruses, especially in infants, the elderly, and people with heart and lung diseases. In an average year in the United States, over 30,000 people die of the flu. Some types of influenza can be prevented by immunization, commonly called a "flu shot." Sometimes, antiviral medications can help reduce the severity of the illness for those already infected. Currently, such medications are given only to groups of people at high risk for complications from influenza.
pneumonia
In people who are elderly, or who have heart disease or breathing problems, flu may develop into pneumonia (noo MOHN yuh), a serious infection of the lungs. Many people die each year from pneumonia, which can be caused by viruses, bacteria, or even fungi.
hepatitis
A group of viruses that infect the liver can cause hepatitis (hep uh TY tis), or inflammation of the liver. Because the liver is important to so many bodily functions, hepatitis is a serious disease. Symptoms of hepatitis include fever, nausea, pain in the abdomen, and jaundice (JAWN dis), or yellowing of the skin. People with hepatitis need medical care. The most common types of hepatitis are identified as A, B, or C.
Hepatitis A is transmitted in human wastes and in contaminated water and food. Illness begins about four weeks after exposure, and recovery takes several weeks. A vaccine for hepatitis A can effectively prevent the disease.
Hepatitis B is more severe than type A. The virus can be transmitted in blood or during sexual contact. It can also be transmitted during tattooing or body piercing if tools are not properly sterilized. Over a million Americans carry hepatitis B, for which there is a vaccine.
Hepatitis C is also more severe than type A. Like hepatitis B, it can be transmitted in blood, during sexual contact, or during tattooing or body piercing. Hepatitis C is the number one reason for liver transplants in the United States. Approximately three million Americans carry hepatitis C.
treating viral diseases
In most cases, there is no particular medicine that can cure a viral infection. Antibiotics, for instance, are only effective against bacteria�not viruses. Although antiviral medications may shorten the length of infection in some cases, the best treatments for viral infections are rest, a well-balanced diet, and plenty of fluids. In addition, many over-the-counter medicines�those available without a prescription�can treat the symptoms of viral infections. They may make you feel better, but they do not cure the infection.
getting healthy, staying healthy
Infectious diseases are unpleasant at best, but they are part of life. When you do get sick, you can help your body recover by going to bed and resting. This treatment and well-balanced meals are all that you need to recover from most mild infections. However, what if you have a more serious infection? Do you know when to see a doctor?
when to seek medical care
If you are worried about your health for any reason, see a doctor and discuss your concerns with him or her. You need to seek professional medical care if you have any of the conditions listed below:
An extremely sore throat, earache, vomiting, diarrhea, or a temperature of 101�F that lasts more than two days
Mucus from your nose or throat that is thick and yellowish green
Difficulty breathing, or severe pain anywhere
A cut, scrape, or sore that does not seem to be healing as it should
An illness that lasts longer than usual
preventing infectious diseases
You can protect yourself from infectious diseases in three ways: avoiding contact with pathogens, making sure that your immunizations are current, and choosing healthful behaviors. But as you know, it is impossible to avoid all pathogens, and immunizations are not available for all infectious diseases. However, choosing healthful behaviors�the best long-term strategy for preventing disease�is something you can always do.
Here are some healthful behaviors you should practice to help you avoid disease.
Wash your hands several times a day, especially before eating and after using the bathroom.
Do not share items that can transfer pathogens, such as towels, eating utensils, cups, or hairbrushes.
Cook and store foods properly. Meats should be cooked thoroughly. Hot foods should be kept hot, and cold foods should be kept cold.
Avoid close contact with people who are ill.
Stay home when you are not feeling well.
Learn to manage stress in healthful ways, and get at least eight hours of sleep each night.
Eat well-balanced meals, and do not skip meals. Exercise regularly, at least three or more times a week.
Avoid unhealthful substances, such as tobacco, alcohol, and illegal drugs.
diabetes
More than 14 million Americans are affected by diabetes. Diabetes (dy uh BEE teez) is a disease in which the body's ability to use glucose (blood sugar) is impaired. Diabetes involves insulin (IN suh lin), a hormone produced by the pancreas that stimulates body cells to take up and use blood sugar.
If not controlled, diabetes can be life-threatening. People with diabetes are also at risk for heart disease, stroke, kidney disease, blindness, infections requiring amputation, and complications during pregnancy. There are two common types of diabetes, type 1 and type 2.
type 1 diabetes
About 5 to 10 percent of diabetics have insulin-dependent diabetes, or type 1 diabetes. A person with type 1 diabetes produces little or no insulin. Without insulin, glucose levels in the blood remain high. Symptoms include thirst, frequent urination, nausea, hunger, fatigue, and weight loss.
Although it can strike at any age, type 1 diabetes usually first appears in children. Type 1 diabetics must monitor their blood glucose levels and give themselves doses of insulin on a strict schedule. In addition, they need to eat the proper amounts of carbohydrates and other nutrients on a regular schedule.
Taking too much insulin, missing a meal, or exercising too much can result in low blood sugar levels. The opposite condition�high blood sugar�occurs when too little insulin is taken or too much food is eaten. If not treated promptly, both of these conditions can be life-threatening.
type 2 diabetes
Also known as noninsulin-dependent diabetes, type 2 diabetes occurs mostly in people over the age of 30. However, it can strike children and teens, especially if they are overweight and do not get enough exercise. People with type 2 diabetes produce sufficient insulin, but their body cells do not respond normally to insulin. As with type 1 diabetes, the result is a high level of glucose in the blood.
Type 2 diabetes usually develops slowly and often goes undetected until symptoms become severe. Symptoms are the same as for type 1 diabetes, but also include drowsiness, itching, blurred vision, numbness in the hands or feet, and frequent, hard-to-heal infections. Risk factors for type 2 diabetes include a family history of diabetes, being overweight, and a lack of physical activity.
Fortunately, many people can prevent type 2 diabetes by maintaining a desirable body weight and by exercising regularly. Some people who develop the disease can control it if they follow a weight-loss and exercise program. In other cases, medications may be used. Excessive sugar consumption is not thought to be a direct cause of type 2 diabetes. However, a high-sugar diet can lead to obesity, which can increase the risk of developing type 2 diabetes.
allergies and asthma
Some people's bodies overreact to harmless substances, and as a result, they may feel sick. An allergy is a disorder in which the immune system is overly sensitive to a particular substance not normally found in the body. One type of allergic reaction can lead to a condition called asthma.
causes of allergies
Allergies develop when foreign substances enter the body and set off a series of reactions. Any substance that causes an allergy is called an allergen. Common allergens include plant pollen, dust, molds, some foods, and even some medicines. Allergens may get into your body when you inhale them, eat them in food, or touch them with your skin.
The immune system's overly sensitive response to an allergen causes large quantities of a chemical called histamine (HIS tuh meen) to be released. Histamine is responsible for the symptoms of an allergy, such as sneezing and watery eyes. Antihistamines�medicines that interfere with the action of histamine�may relieve symptoms. However, the best strategy is to try to avoid any substance to which you are allergic.
Some allergic reactions can be very serious and require immediate medical attention. For example, people who are severely allergic to bee stings must carry medicine with them in case they get stung.
asthma
Asthma is a disorder in which a person's respiratory passages become inflamed and narrow significantly in reaction to certain "triggers."
Asthma attacks can be triggered by many things, including certain allergens, stress, cold weather, tobacco smoke, or exercise. During an attack, a person may wheeze, cough, or become short of breath. Some people describe an asthma attack as trying to breathe through a straw.
managing asthma
Approximately 15 million people in the United States have asthma, and nearly 6 million of those are under 18. Asthma can be serious, but it can be managed so that asthma sufferers can lead normal, active lives.
Managing asthma involves avoiding the triggers that bring on asthma attacks. It may also involve the use of medicines. One medicine, called a "controller," is taken on a daily basis to prevent asthma attacks from occurring. Another medicine, called a "rescue drug," is taken when someone has an asthma attack. Rescue drugs relax the muscles in the airways, making it easier to breathe.
arthritis
Inflammation or irritation of a joint is known as arthritis (ahr THRY tis). Arthritis is one of the most common chronic diseases, affecting nearly one in every three adults in the United States. In fact, arthritis is the leading cause of disability among Americans over age 15.
Arthritis is not life-threatening, but it can be extremely painful and disabling. Arthritis results in joint stiffness, joint pain, or swelling in one or more joints. There is no cure for most types of arthritis. However, treatments and exercise can reduce the severity of the symptoms.
osteoarthritis
The most common type of arthritis is osteoarthritis (ahs tee oh ahr THRY tis). This form of arthritis is caused by wear and tear on a joint after years of use or by repeated injuries to a joint. Most people who live past 60 will develop some form of osteoarthritis.
Symptoms of osteoarthritis develop slowly, usually beginning as a mild ache or soreness. Osteoarthritis can occur in almost any joint, but most commonly occurs in hips, knees, spine, and fingers. When it occurs at the finger joints, bony growths often appear.
Treatment for osteoarthritis may involve drugs, heat and cold treatments, and exercise. Exercise is important to maintain joint flexibility. Sometimes, weight loss is recommended to ease stress on the joints. In severe cases of osteoarthritis, surgery may be required to repair or replace affected joints.
rheumatoid arthritis
In rheumatoid arthritis (ROO muh toyd), the membrane surrounding a joint becomes inflamed. The inflammation then spreads to other areas of the joint. An affected joint becomes hot, red, and swollen. Areas of the body other than joints may also become inflamed.
Rheumatoid arthritis affects both the young and the old. Over two million Americans suffer from rheumatoid arthritis. The exact cause of rheumatoid arthritis is still unclear. Much evidence suggests that the immune system malfunctions and attacks some of the body's own tissues. This "self-attack" leads to the inflammation that is characteristic of rheumatoid arthritis.
Any joint in the body may be affected by rheumatoid arthritis, although joints in the wrist and knuckles are most commonly affected. If not treated, rheumatoid arthritis can cause joints to stiffen in deformed positions. The damage can be so severe that it changes the shape of the joint.
Treatment includes aspirin or other anti-inflammatory drugs, exercise, and rest. Early diagnosis and treatment by a doctor are the best ways to reduce the severity of the disease.