I NFECTION AND I MMUNITY

There are two basic types of disease: ones that are infectious, or extrinsic, meaning that they are contagious or communicable and can be spread by contact between people, and ones that are intrinsic, or not infectious. Diseases in general and noninfectious diseases in particular are discussed in essays devoted to those subjects. So, too, is infection itself, a subject separate from infectious diseases: a person can get an infection, such as tetanus or salmonella, without necessarily having a disease that can be passed on through contact with others in the same way that colds, malaria, or syphilis is spread.

The background on scientists' progressive understanding of the microorganisms that cause disease and the means of fighting these microorganisms are discussed in Infection. Among the leading figures in that history were the French chemist and microbiologist Louis Pasteur (1822-1895) and the German bacteriologist Robert Koch (1843-1910), who contributed greatly to what is known today as germ theory—the idea that infection and infectious diseases are brought about by microorganisms. In most cases, the organisms are too small to be seen with the naked eye. They include varieties of amoeba and worm, discussed in the essay Parasites and Parasitology, as well as viruses and some forms of bacteria and fungi, which together are known as pathogens, or disease-carrying parasites. Other terms related to infectious diseases, their agents, and the prevention and study of them are defined in the essay Infection.

IMMUNE MECHANISMS.

The human body has numerous mechanisms for protecting itself from infectious disease, the first line of defense being the skin. Skin shields us all the time from unseen attackers and generally is able to prevent pathogens from entering the body; however, any break in the skin, such as a cut or scrape, provides an opening for microorganisms to invade the body. Germs that normally would be prevented from entering the body are able to invade the bloodstream through such openings. This is why it is so very important, in any situation involving potential contact with infection, to protect the skin. With the advent of AIDS, doctors and members of other professions who are likely to touch people carrying diseases—including officers arresting addicts or prostitutes—are much more likely to do their work wearing heavy plastic gloves.

Suppose that a microorganism makes it through the barrier of skin, thanks to a cut or other opening; if so, the body puts into action a second defensive mechanism, the immune system. This system is a network of organs, glands, and tissues that protects the body from foreign substances. Without a properly functioning immune system, a person could die simply by walking out the door in the morning and coming into contact with an airborne infectant. Even in relatively healthy people, the immune system may be unable to react adequately to an invasion of microorganisms. In such cases, disease develops.

T RANSMISSION OF D ISEASES

Infectious diseases, by definition, are transmitted easily from one person to another. We have all been told, for instance, not to drink after someone who has a cold. On a much more serious level, persons who are sexually active or potentially sexually active, but not settled in a monogamous (one-partner) relationship, are advised to avoid unprotected sexual contact so as not to contract AIDS or some other sexually transmitted disease (STD). In these and many other cases, microorganisms travel from the carrier of the disease to the uninfected person. (Actually, in the case of AIDS, the pathogen is a virus, which is not, strictly speaking, an organism or even a living thing; however, viruses usually are lumped in with bacteria, amoeba, and some fungi as microorganisms.)

Pathogens can be spread by many methods other than direct contact, including through water, food, air, and bodily fluids—blood, semen, saliva, and so on. For instance, any time a person with an infection coughs or sneezes, they may be transmitting illness. This is how diseases such as measles and tuberculosis are passed from person to person. AIDS and various STDs, as well as many other conditions, such as hepatitis, are transferred when one person comes into contact with the bodily fluids of another. This is the case not only with sexual intercourse but also with blood transfusions and any number of other interactions, including possibly drinking after someone. (Contrary to rumors that circulated in the early 1980s, when AIDS first made itself known, that particular syndrome cannot be transferred by saliva, but the common cold and other viral infections can be.)

Cholera, caused by a bacterium found in dirty wells and rivers from India to England (in the 1800s, at least), is an example of a waterborne disease. Many foodborne pathogens tend to bring about what would be more commonly thought of as an illness than a disease, since in everyday language the latter term implies a long-term affliction, whereas food poisoning usually lasts for a week or so. (Still, some forms of food poisoning can be fatal.) Bacterial contamination may occur when food is not cooked thoroughly, is left unrefrigerated, is prepared by an infected food handler, or otherwise is handled in an unsanitary or improper fashion. (The case of Typhoid Mary, discussed near the conclusion of this essay, is an extreme example of this form of transmission.)

Additionally, diseases may be transferred by vectors—animals (usually insects) that carry microorganisms from one person to another. Vectors may spread a disease either by mechanical or by biological means. Mechanical transmission occurs, for example, when flies transfer the germs for typhoid fever from the feces (stool) of infected people to food eaten by healthy people. Biological transmission takes place when an insect bites a person and takes infected blood into its own system. Once inside the insect's gut, the disease-causing organisms may reproduce, increasing the number of parasites that can be transmitted to the next victim. This is how the Anopheles mosquito vector, for instance, transfers malaria.