Infectious diseases are usually caused by microorganisms that invade the body and multiply. There are many types of infectious organisms (see also Overview of Infectious Disease).
The following are some examples of how microorganisms can invade the body:
- Through the mouth, eyes, or nose
- Through sexual contact
- Through wounds or bites
- Through contaminated medical devices
People can ingest microorganisms by swallowing contaminated water or eating contaminated food. They may inhale spores or dust or inhale contaminated droplets coughed or sneezed out by another person. People may handle contaminated objects (such as a doorknob) or come into direct contact with a contaminated person and then touch their eyes, nose, or mouth.
Some microorganisms are spread through body fluids such as blood, semen, and stool. Thus, they can invade the body through sexual contact with an infected partner. They also can enter through nonsexual contact with body fluids, such as while providing personal care or medical services.
Human and animal bites and other wounds that break the skin can allow microorganisms to invade the body. Infected insects and ticks can spread diseases when they bite.
Microorganisms can also adhere to medical devices (such as catheters, artificial joints, and artificial heart valves) that are placed in the body. Microorganisms may be present on the device when it is inserted if the device was accidentally contaminated. Or infecting organisms from another site may spread through the bloodstream and lodge on an already implanted device. Because implanted material has no natural defenses, the microorganisms can easily grow and spread, causing disease.
After invading the body, microorganisms must multiply to cause infection. After multiplication begins, one of three things can happen:
- Microorganisms continue to multiply and overwhelm the body’s defenses.
- A state of balance is achieved, causing chronic infection.
- The body—with or without medical treatment—destroys and eliminates the invading microorganism.
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 the following:
- Whether the microorganism produces toxins, enzymes, or other substances
- Whether it develops resistance to antimicrobial drugs
- Whether it can block the body's defense mechanisms
- How well the person's immune system is functioning
Many disease-causing microorganisms have properties that increase the severity of the diseases they cause (virulence) and that help them resist the body’s defense mechanisms. These properties include the following:
- Ways to block the body's defenses
Production of toxins and enzymes
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 diseases such as tetanus, toxic shock syndrome, botulism, anthrax, and cholera.
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.
Blockage of the body's defenses
Some microorganisms have ways of blocking the body’s defense mechanisms, such as the following:
- Interfering with the body’s production of antibodies or T cells (a type of white blood cell), which are specifically armed to attack the microorganisms
- Being enclosed in protective outer coats (capsules) that prevent white blood cells from ingesting the microorganisms (for example, the fungus Cryptococcus, which actually develops a thicker capsule after it enters the lungs for the specific purpose of resisting the body’s defenses)
- Resisting being split open (lysed) by substances circulating in the bloodstream
- Producing substances that counter the effects of antibiotics
Some bacteria can produce a layer of slime (called biofilm) that helps them attach to cells and to foreign material such as intravenous catheters, suture material, and medical implants and devices. The biofilm protects bacteria from being ingested by immune cells and being killed by antibiotics.
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 (called antibiotic resistance).
Malfunction of the immune system
If the immune system is not functioning well (called immunocompromise), people are more susceptible to infections. The immune system may not function well because of the following:
- People are born with a hereditary disorder that impairs the immune system (an immunodeficiency disorder).
- A disorder that is acquired later (such as HIV infection or cancer) weakens it.
- People need to take a drug that suppresses the immune system (immunosuppressants), such as those used to prevent a transplanted organ from being rejected, or corticosteroids, used to reduce inflammation.