Everyone including healthy people carries some microorganisms on the skin or in the body, but normally the body’s natural defenses prevent these from causing harm.
Infections occur when the body is invaded by disease-causing organisms bacteria, viruses, fungi, protozoa, or larger metazoa, such as tapeworms. They may enter the body by various means: through the nose or mouth; through a break in the skin; or through physical contact with an infected person or thing, as in sexual or skin infections.

Some infections, such as a common cold, are minor and short-lived; others, such as athlete’s foot, are more persistent, but are still not life-threatening. Certain infections, however, for example those that cause AIDS, meningitis, poliomyelitis, typhus, and rabies, are extremely dangerous. Infections by protozoa or larger organisms are more properly called infestations, and are caused by organisms that live on humans as parasites.

Bacteria have a variety of forms, several of which are shown above. Among these there are three main types— cocci, which are roughly spherical or oval; bacilli, which are elongated, or rod-shaped; and spirilli, which have a spiral shape. Size varies considerably, but most bacteria are between 0.2 X 10 3 and 2.0 X 10~3 millimeters in length. The photo-micrograph is of a Salmonella bacterium, the type that causes various infections of the gastrointestinal tract, including typhoid.

Agents of infection

The most common infective agents (pathogens) are bacteria. Many forms live harmlessly in the human body, particularly in the lower digestive tract, but others cause minor or severe infections such as boils, tonsillitis, and pneumonia.

Viruses are smaller than bacteria, and can be seen only with the aid of an electron microscope. Viral infections include the common cold, chickenpox (varicella), poliomyelitis, and herpes and AIDS.

Rickettsias are unusual agents of infection found on fleas and lice; they have characteristics in common with both bacteria and viruses, and can transmit diseases such as typhus to humans.
Fungi are plantlike organisms that can cause various diseases, including ringworm and thrush the common name for moniliasis or candidiasis.

Protozoa are single-celled parasites that cause diseases such as malaria, amebic dysentery, and toxoplasmosis. Metazoa are manycelled parasites, such as tapeworms and lice.

Fertilized hen’s eggs are used as a living culture medium in which to grow viruses for making vaccines. Here eggs are being injected with a strain of influenza virus.

The spread of disease

Diseases spread in various ways. Poliomyelitis and cholera are transmitted by contaminated water. Many viruses, such as those causing chickenpox, are spread by airborne droplets that are sneezed, coughed, or breathed out by someone with the disease. Diseases such as gonorrhea, syphilis, and AIDS (Acquired Immune Deficiency Syndrome), which are transmitted by sexual contact, are known as sexually transmitted diseases.

Infectious organisms can be contained in the saliva, sputum, vomit, blood, pus, or excreta (waste products) of an infected person. Food may be a carrier of disease, particularly if it is neither fresh nor freshly cooked, or if it is contaminated with animal- or insect-borne diseases such as tapeworm or salmonella.

Some diseases are caught directly from animals. For example, psittacosis can be caught from certain birds, and rabies is transmitted through the saliva of infected mammals.
Open wounds can become infected by any bacteria that come into contact with them. Most serious of such infections is tetanus, caused when tetanus bacilli which are commonly found in soil enter the body through a dirty wound. Tetanus can be prevented by vaccination. If the disease develops, however, it can be fatal.

Chicken pox, an infectious disease caused by a virus, causes a rash on the face and body. Contracting the disease usually confers immunity for life.

Prevention of disease

Personal hygiene is the key to preventing the spread of many diseases. Important principles of hygiene include: washing the hands after going to the toilet and before handling food; covering any wounds before handling food; keeping a sickroom clean, and disinfecting or destroying any soiled handkerchiefs or dressings; and washing a sick person’s dishes, eating utensils, and clothes separately from those of the rest of the family.

It is also important to take extra care of hygiene when traveling abroad because of the presence of infective agents to which the body is unaccustomed, and to have any recommended inoculations before starting a trip. Keeping pets and farm animals as clean as possible, and washing the hands after handling them particularly before touching food or anything that might come into contact with it is another good principle. In any case, it is sensible for each member of a household to have his or her own facecloth, toothbrush, and hairbrush.

The body has various systems for preventing and fighting infections. The skin prevents many organisms from entering the body, and blood clots and tissue repairs soon seal any minor wounds in the skin. The tonsils, adenoids, and mucous membranes of the nose and throat help to trap any inhaled germs. The lymph nodes together with the spleen manufacture antibodies against infections that penetrate the outer defenses, and the liver can destroy various disease-produced toxins. White blood cells attack invading germs, and blood also contains antibodies, produced by white cells in response to infection, which provide protection (immunity) against further attacks by the same infecting agents.

Immunity is mostly developed by the body’s natural immune system, but some immunity can be conferred. Newborn babies inherit antibodies from their mothers and acquire others from mother’s milk. Others again are developed in response to specific infections. Immunity can also be induced artificially by vaccination, which involves introducing dead or weakened disease agents into the body to stimulate the formation of antibodies.

Curing infections

Antibiotics are the most successful agents that mankind has created to combat infectious diseases. Most work only against bacterial infections, however, such as tonsillitis and pneumonia.

Viruses are far more resistant to known drugs. The symptoms of viral diseases can be treated for instance, decongestants relieve the symptoms of colds but generally viral infections must be left to take their natural course. However, sometimes drugs are used to slow the course of serious viral infections. For example, AZT seems to slow the progress of AIDS.

Fungicidal drugs can ease such conditions as athlete’s foot, and ultraviolet radiation from sunlight or from a sunlamp helps to kill the germs that are present in acne. Certain drugs are effective in eliminating parasites, such as threadworms and tapeworms. When drugs fail or are inappropriate, however, surgery is also a possible treatment for instance, to remove an infected appendix.

Schistosomiasis, formerly known as bilharziasis, is a disease caused by Schistosoma parasites, which are common in tropical America, Asia, and Africa. The parasite also depends on some types of water snail, and people become infected from water in which these snails live. The parasite’s life cycle has four phases: one in snails, one in humans, and two intermediate stages in water, from which both hosts acquire the infestation.