Infectious diseases are usually caused by microorganisms that invade the body and multiply.
Invasion by most microorganisms begins when they adhere to cells in a person's body. Adherence is a very specific process, involving "lock-and-key" connections between the microorganism and cells in the body. Being able to adhere to the surface of a cell enables microorganisms to establish a base from which to invade tissues.
Whether the microorganism remains near the invasion site or spreads to other sites and how severe the infection is depend on such factors as whether it produces toxins, enzymes, or other substances, whether it develops resistance to antimicrobial drugs, whether it can block the body's defense mechanisms, and how well the person's immune system is functioning.
Some microorganisms that invade the body produce toxins. For example, the bacteria Clostridium tetani in an infected wound produce a toxin that causes tetanus. Some diseases are caused by toxins produced by microorganisms outside the body. For example, staphylococci bacteria living in food may produce a toxin that causes food poisoning when that food is eaten, even if the staphylococci have been killed. 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 cause infection. After multiplication begins, one of three things can happen:
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 through tissues faster. Other bacteria produce enzymes that allow them to enter and/or pass through cells.
Some microorganisms have ways of blocking the body's defense mechanisms (see Bacterial Defenses), such as the following:
Some bacteria can produce a layer of slime (called biofilm) that helps them attach to cells and to foreign material such as IV catheters, suture material, and medical implants and devices. The biofilm protects bacteria from being ingested by immune cells and being killed by antibiotics (see Biofilm).
Microorganisms that do not at first have ways of blocking the body's defenses sometimes develop them over time. For example, some microorganisms, after being repeatedly exposed to penicillin, become resistant to that drug (see Antibiotic Resistance).
The immune system may not function well because
Last full review/revision October 2012 by Allan R. Tunkel, MD, PhD