How Infection Develops
Infectious diseases are usually caused by microorganisms that invade the body and multiply. Invasion by most microorganisms begins when they adhere to a person's cells. Adherence is a very specific process, involving "lock-and-key" connections between the microorganism and cells in the person's body. Whether the microorganism remains near the invasion site or spreads to other sites depends on such factors as whether it produces toxins, enzymes, or other substances.
Some microorganisms that invade the body produce toxins. For example, Clostridium tetani in an infected wound produces a toxin that causes tetanus. Some illnesses are caused by toxins produced by microorganisms outside the body. Food poisoning caused by staphylococci is one example. Most toxins contain components that bind specifically with molecules on certain cells (target cells). Toxins play a central role in such diseases as tetanus, toxic shock syndrome, botulism, anthrax, and cholera.
After invading the body, microorganisms must multiply to produce infection. After multiplication begins, one of three things can happen: the microorganisms can continue to multiply and overwhelm the body's defenses; a state of balance can be achieved, producing a chronic infection; or the body--with or without medical treatment--can destroy and eliminate the invading microorganism.
Many disease-causing microorganisms have properties that increase the severity of the diseases they cause (virulence) and help them resist the body's defense mechanisms. For example, some bacteria produce enzymes that break down tissue, allowing the infection to spread faster.
Some microorganisms have ways of blocking the body's defense mechanisms. For example, a microorganism may be able to interfere with the body's production of antibodies or T cells (a type of white blood cell) specifically armed to attack them. Others have outer coats (capsules) that resist being ingested by white blood cells. The fungus Cryptococcus actually develops a thicker capsule after it enters the lungs for the specific purpose of resisting the invaded body's defenses. Some bacteria resist being split open (lysed) by substances circulating in the bloodstream. Some even produce substances that counter the effects of antibiotics.
 See the sidebar Identifying an Infectious Organism.
See the sidebar Biological Warfare and Terrorism.
See the sidebar Infection From Medical Devices.
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