What is an infection?

Process and Effects

Healthy people live with potential pathogens; that is, people have on and in their bodies non-disease-causing bacteria. They live in harmony with these organisms and in fact benefit from their presence. For example, some of the bacteria found in the intestinal tract supply vitamin K, which is important in blood-clotting reactions.

The human body has several features that prevent disease-causing organisms from inducing an infection. These features include anatomical barriers, such as unbroken skin, and the mucus in the nose, mouth, and lungs, which can trap pathogens. Another defense is the acid within the stomach, and even bacteria that are normally present in certain areas of the body can force out more harmful bacteria. The
immune system is specially developed to ward off intruders.

Immune cells and factors secreted from these cells provide the next line of defense against invading organisms. Antibodies are secreted from specialized white blood cells known as plasma cells. These antibodies are very specific in their recognition of pathogens. For example, an antibody may recognize one particular strain of bacteria but not another. Antibodies attach themselves to the part of the bacterium called the antigen. Once bound to the antigen, they aid in the destruction of the pathogen. In addition to plasma cells, other white blood cells help in combating infections, including two types of phagocytes called macrophages and neutrophils. Both of these types of immune cells have the ability to eat and digest pathogens such as bacteria in a process known as phagocytosis.

In order to cause disease, microorganisms must somehow overwhelm the body’s natural defenses and immune system. Bacteria capable of causing infections may even be naturally occurring organisms in the body that have left their normal environment and overcome the elements that usually hold them at bay. For example, some bacteria that typically reside in the mouth may cause pneumonia (inflammation of the lungs) if they gain access to the lungs.

Other infections, such as a cold, the flu, or a sexually transmitted disease, can be caused by pathogens that do not normally reside in the body. These kinds of infections are called communicable or transmissible infections. A physician treating someone who has been bitten by a bat, skunk, or dog will want to know whether the animal has rabies, as rabies is a viral infection that is transmitted via a bite that breaks the skin and contaminates the wound with infectious saliva.

No matter what the route of infection, the body must mount a response to the intruding microorganism. Often the observable signs and symptoms of an infection are not caused by the direct action of the infecting pathogen but rather reflect the immune system’s response to the pathogen. The most frequent signs and symptoms include inflammation and pain at the site of infection, as well as fever.

The inflammatory response is a nonspecific defense that is triggered whenever body tissues are injured, as in the case of infection. The goals of this response are to prevent the spread of the infectious agent to nearby tissues, destroy the pathogens, remove the damaged tissues, and begin the healing process. Signs of inflammation include redness, edema (tissue swelling), heat, pain, and loss of normal function. At first glance, these reactions do not appear to be beneficial to the body, but they do help fight the infection and aid in the healing process. The redness is attributable to an increase in blood flow to the area of infection, which helps provide nutrients to the tissue and remove some of the waste products that develop as the immune system fights the infection. With this increase in blood flow comes an increase in the temperature and amount of blood that leaks out of blood vessels into the tissue spaces, causing
edema at the site of infection. Some of the blood that leaks into the site of infection contains clotting proteins that help form a clot around the infected area, thereby reducing the chances that the pathogen could escape into the bloodstream or uninfected tissue nearby. Pain is present when the damaged
tissue releases waste products and the pathogen releases toxins. The swelling of the injured area and the pain associated with infections keep the patient from using that area of the body and thus aid in healing. While some painkillers such as aspirin reduce the inflammatory reaction by stopping the production of some of the chemicals released during inflammation, these drugs do nothing to harm the pathogen; they only block the body’s response to the microorganism.

Some of the same chemicals that are found in inflamed tissues also cause fever, an abnormally high body temperature that represents the body’s response to an invading microorganism. The body’s thermostat, located in a region of the brain known as the hypothalamus, is set at about 37 degrees Celsius (about 99 degrees Fahrenheit). During an infection, the thermostat is reset to a higher level. Chemicals called
pyrogens are released from macrophages. Once again, aspirin-like drugs can be used to reduce the fever by inhibiting the action of some of these chemicals in the hypothalamus.

The body responds to
viral infections in a similar way. Virally infected cells secrete interferon, exerting an antiviral action that may provide protection to uninfected neighboring cells. It appears that interferon acts to inhibit the virus from replicating. Cells that are already infected must be destroyed to rid the body of the remaining virus.

In addition to being part of the inflammatory response, certain white blood cells play an important role in attempting to remove the pathogen. Soon after inflammation begins, macrophages already present at the site of infection start to destroy the microorganism. At the same time, chemicals are being released from both the damaged tissue and the macrophages to recruit other white blood cells, such as neutrophils. The neutrophils, like the macrophages, are effective at attacking and destroying bacteria; unlike the macrophages, they often die in the battle against infection. Dead neutrophils are seen as a white exudate called pus.

Other manifestations of infection include systemic (whole-body) effects as well as changes at the site of infection. As noted, fever is a systemic effect mediated by chemicals from the site of infection. Some of these same factors have the ability to act on the bone marrow to increase the production of white blood cells. Physicians look for fever and an increase in the number of white blood cells as signs of infection. If the infection is severe, then the bone marrow may not be able to keep up with the demand, and an overall decrease in white blood cell number will be found.

Complications and Disorders

Physicians and other health-care workers must use patient history, signs and symptoms, and laboratory tests to determine the type of infection and the most appropriate treatment. Patient history can often tell the examiner how and when the infection started. For example, the patient may have cut himself or herself, been exposed to someone with an infectious disease, or had intimate contact with someone carrying a sexually transmitted disease. Signs of infection—edema, pain, fever—are usually easy to detect, but some symptoms may be rather vague, such as feeling tired and weak. These general signs and symptoms may indicate to a physician that the patient has an infection, but they will not provide information about the type of microorganism causing the infection. Nevertheless, some microorganisms do cause specific symptoms. For example, the varicella-zoster virus that causes chicken pox and the paramyxovirus that causes measles leave characteristic rashes.

When the microorganism does not have a characteristic sign, physicians must use laboratory tests to determine the pathogen involved. Diagnosis of the disease relies on identifying the causative pathogen by microscopic examination of a specimen of infected tissue or body fluid, by growing the microorganism using culture techniques, or by detecting antibodies in the blood that have developed against the pathogen.

Once the physician determines what type of microorganism has caused the infection, he or she will have to determine the best treatment to eradicate the disease. Drug therapy usually consists of antibiotics and other antimicrobial agents. The selection of the appropriate drug is important, as certain pathogens are susceptible only to certain antibiotics. Unfortunately, few effective antiviral drugs are available for many infectious viruses. In these cases, drugs are mainly used to treat symptoms such as fever, pain, diarrhea, and vomiting rather than to destroy the virus.

Anti-infective drugs are commonly used by physicians to treat infections. Agents that kill or inhibit the growth of bacteria are known as antibiotics and can be applied directly to the site of infection (topical), given by mouth (oral), or injected. The latter two modes of administration allow the drug to be carried throughout the body by way of the blood. Some antibiotic drugs are effective against only certain strains of infectious bacteria. Antibiotics that act against several types of bacteria are referred to as broad-spectrum antibiotics. Some bacteria develop resistance to a particular antibiotic, requiring the physician to switch agents or use a combination of antibiotics. Antibiotic therapy for the treatment of infections should be used when the body has been invaded by harmful bacteria, when the bacteria are reproducing at a more rapid rate than the immune system can handle, or to prevent infections in individuals with an impaired immune system.

Some serious common
bacterial infections include gonorrhea, which is sexually transmitted and treatable with penicillin; bacterial meningitis, which causes inflammation of the coverings around the brain and is treatable with a variety of antibiotics; pertussis (whooping cough), which is transmitted by water droplets in air and treatable with erythromycin; pneumonia, which causes shortness of breath, is transmitted via the air, and can be treated with antibiotics; tuberculosis, which infects the lungs and is treatable with various antibiotics; and salmonella, which is transmitted in food or water contaminated with fecal material, causes fever, headaches, and digestive problems, and is treatable with antibiotics. It should be noted that antibiotics are not effective in viral infections; only bacterial infections are treated with antibiotics.

Antiviral drugs such as acyclovir, amantadine, and zidovudine (formerly known as azidothymide, or AZT) are used in the treatment of infection by a virus. These drugs have been difficult to develop. Most viruses live within the cells of the patient, and the drug must in some way kill the virus without harming the host cells. In fact, antiviral agents cannot completely cure an illness, and infected patients often experience recurrent disease. Nevertheless, antiviral drugs do reduce the severity of these infections.

There are several common viral infections. Human immunodeficiency virus (HIV) infection, which causes Acquired immunodeficiency syndrome (AIDS), is transmitted by sexual contact or contaminated needles or blood products; it is often treated with zidovudine but remains lethal. Chicken pox (varicella-zoster virus), which is transmitted by airborne droplets or direct contact, is treated with acyclovir. The common cold is caused by numerous viruses that are transmitted by direct contact or air droplets and has no effective treatment other than drugs that reduce the symptoms. Hepatitis is transmitted by contaminated food, sexual contact, or blood; it causes flulike symptoms and jaundice (a yellow tinge to the skin caused by liver problems) and may be helped with the drug interferon. Influenza viruses are transmitted by airborne droplets; the only treatment for the flu is of its symptoms. Measles is transmitted by virus-containing water droplets and causes fever and a rash; treatment consists of alleviating the symptoms. Mononucleosis is transmitted via saliva and causes swollen lymph nodes, fever, a sore throat, and generalized tiredness; a patient with mononucleosis can receive treatment only for the symptoms, as no cure is available. Poliomyelitis (polio) is transmitted by fecally contaminated material or airborne droplets and can eventually cause paralysis; no treatment is available. Rabies is caused by a bite from an infected animal, as the virus is present in the saliva; the major symptoms include fever, tiredness, and spasms of the throat. Treatment for rabies must be given before these symptoms have appeared, as after that point, no effective treatment exists, and the virus usually proves fatal. Rubella is transmitted by virus-containing air droplets and is associated with a fever and rash; there is no treatment other than for the symptoms.

A major problem with infectious diseases is that there is almost always a delay between when the microorganism enters the body and the onset of signs and symptoms. This gap may range from a few hours or days to several years. A patient without noticeable symptoms is likely to spread the pathogen, setting up a cycle in which individuals unknowingly infect others who, in turn, pass on the disease-causing agent. Large numbers of people can quickly become infected in this manner.

One way to prevent the spread of infectious agents is to vaccinate patients. Diseases such as diphtheria, measles, mumps, rubella, poliomyelitis, and pertussis are rare in the United States because of an aggressive immunization program. When a patient is vaccinated, the vaccine usually contains a dead or inactive pathogen. After the vaccine is administered, usually by injection, the immune system responds by making antibodies against the antigens on the microorganism. Since the pathogen in the vaccine is unable to cause disease, the patient has no symptoms after immunization. The next time that the person is exposed to the infectious agent, his or her immune system is prepared to fight it before symptoms become evident.

In addition to immunization to prevent infection, individuals can largely avoid serious infectious diseases through good hygiene with respect to food and drink, frequent washing, the avoidance of contact with fecal material and urine, and the avoidance of contact with individuals who are infected and capable of transmitting the disease. When such avoidance is impractical, other protective measures can be taken.

Sexually transmitted diseases
are usually preventable by using barrier contraceptives and practicing safer sex. The most common of these diseases include chlamydial infections, trichomoniasis, genital herpes, and HIV infections. Prevention is particularly important in cases of herpes and HIV, as these are viral infections with no known cures.

Some infections can be acquired at birth, including gonorrhea, genital herpes, chlamydial infections, and salmonella. These microorganisms exist in the birth canal, and some infectious agents can even pass from the mother to the fetus via the placenta. The more serious infections transmitted in this manner are rubella, syphilis, toxoplasmosis, HIV, and the cytomegalovirus. The risk of transmitting these infections can be reduced by treating the mother before delivery or performing a cesarean section (surgical delivery from the uterus), thereby avoiding the birth canal.

Perspective and Prospects

The ancient Egyptians were probably the first to recognize infection and the body’s response to the introduction of a disease-causing microorganism, as some hieroglyphics appear to represent the inflammatory process. Sometime in the fifth century BCE, the Greeks noted that patients who had acquired an infectious disease and survived did not usually contract the same illness a second time.

More solid scientific evidence about infections was provided in the nineteenth century by
Edward Jenner, an English physician and scientist. Jenner was able to document that milkmaids seemed to be protected against smallpox because of their exposure to cowpox. With this knowledge, he vaccinated a boy with material from a cowpox pustule. The boy had a typical inflammatory response, and he showed no symptoms of smallpox
after being injected with the disease a few months later. His immune system protected him from the virus. Since that time, scientists have reached a much better understanding of how the body deals with infection.

Many scientists are focusing their attention on how pathogens are transmitted from the source of infection to susceptible individuals. Epidemiology is the study of the distribution and causes of diseases that are prevalent in humans. Since some infectious diseases are communicable (transmittable), epidemiologists gather data when an outbreak occurs in a population. These data include the source of infectious agents (the tissues involved), the microorganisms causing the disease, and the method by which the pathogens are transmitted from one person to another. Physicians and other health-care workers help in the battle against infections by identifying susceptible individuals; developing and evaluating sources, methods, and ways to control the spread of the pathogens; and improving preventive measures, which usually include extensive educational efforts for the general population. With this knowledge, scientists and physicians attempt to eradicate the disease.

While scientists and physicians have made great advances in the understanding of infection, many problems remain. The spread of certain diseases, such as sexually transmitted diseases, is difficult to control except by modifying human behavior. The most difficult to treat are viral illnesses for which the drugs that are used are ineffective in completely eradicating the virus and bacterial diseases for which the bacteria have developed drug resistance. Because these microorganisms evolve rapidly, new strains continually emerge. When a new infectious agent develops, it is often years before scientists can devise an effective drug or vaccine to treat the disease. In the meantime, large numbers of patients may become ill and even die. Perhaps the most effective way to combat infection is to use preventive measures whenever practical.

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