Infectious diseases are usually caused by microorganisms that invade the body and multiply. Invasion by most microorganisms begins when they adhere to cells in the body. Whether the microorganism remains near the invasion site or spreads to other sites depends on many factors, including whether it produces toxins, enzymes, or other substances. For example, Clostridium tetani in an infected wound produces a toxin that causes tetanus. Food poisoning is caused by toxins produced by staphylococcal organisms that are outside the body. Most toxins contain components that bind specifically with molecules on certain cells (target cells).
After invading the body, microorganisms start to multiply. One of 3 things can then happen: the microorganisms can continue to multiply and overwhelm the body’s defenses, resulting in illness; a state of balance can be achieved, resulting in a chronic infection; or the body—with or without treatment—can destroy and eliminate the invading microorganism.
Many microorganisms that cause disease have properties that increase the severity of the disease 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. Other microorganisms have ways of blocking the body’s defense mechanisms, such as by interfering with production of antibodies or T cells (a type of white blood cell). Others have outer coats (capsules) that resist being ingested by white blood cells. Some bacteria resist being destroyed by substances circulating in the bloodstream. Some even produce substances that counter the effects of antibiotics.