Development of infection

[Greta]

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