Infectious diseases are usually caused by microorganisms that invade the body and multiply. There are many types of infectious organisms Types of Infectious Organisms 
(see also Overview of Infectious Disease Overview of Infectious Disease Microorganisms are tiny living creatures, such as bacteria and viruses. Microorganisms are present everywhere. Despite their overwhelming abundance, relatively few of the thousands of species... read more ).
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 Artificial Joint Infectious Arthritis Artificial joints can become infected by bacteria. Bacteria may infect the artificial joint during or after surgery, causing infection. Symptoms may include pain, swelling, and limited range... read more , and artificial heart valves Infective Endocarditis Infective endocarditis is an infection of the lining of the heart (endocardium) and usually also of the heart valves. Infective endocarditis occurs when bacteria enter the bloodstream and travel... read more 
) 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 Production of toxins and enzymes 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... read more
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:
Toxins
Enzymes
Ways to block the body's defenses
Production of toxins and enzymes
Some microorganisms that invade the body produce toxins. Toxins are chemicals that can harm living things. Toxins can be man-made, such as certain insecticides, or natural, such as those produced by certain organisms. For example, the bacteria Clostridium tetani in an infected wound produce a toxin that causes tetanus Tetanus Tetanus results from a toxin produced by the anaerobic bacteria Clostridium tetani. The toxin makes muscles contract involuntarily and become rigid. Tetanus usually develops after a wound... read more . 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 Staphylococcal Food Poisoning Staphylococcal food poisoning results from eating food contaminated with toxins produced by certain types of staphylococci, resulting in diarrhea and vomiting. This disorder can be caused by... read more 
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 Tetanus Tetanus results from a toxin produced by the anaerobic bacteria Clostridium tetani. The toxin makes muscles contract involuntarily and become rigid. Tetanus usually develops after a wound... read more , toxic shock syndrome Toxic Shock Syndrome Toxic shock syndrome is a group of rapidly progressive and severe symptoms that include fever, rash, dangerously low blood pressure, and failure of several organs. It is caused by toxins produced... read more , botulism Botulism Botulism is an uncommon, life-threatening poisoning caused by toxins produced by the anaerobic bacteria Clostridium botulinum. Botulism toxins, usually consumed in food, can weaken or... read more , anthrax Anthrax Anthrax is a potentially fatal infection with Bacillus anthracis, a gram-positive, rod-shaped bacteria (see figure ). Anthrax may affect the skin, the lungs, or, rarely, the digestive... read more 
, and cholera Cholera Cholera is a serious infection of the intestine that is caused by the gram-negative bacteria Vibrio cholerae and that causes severe diarrhea, which can be fatal without treatment. People... read more .
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 Bacterial Defenses Bacteria are microscopic, single-celled organisms. They are among the earliest known life forms on earth. There are thousands of different kinds of bacteria, and they live in every conceivable... read more , such as the following:
Interfering with the body’s production of antibodies Antibodies One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more
or T cells T cells One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more (a type of white blood cell White Blood Cells The main components of blood include Plasma Red blood cells White blood cells Platelets read more 
), which are specifically armed to attack the microorganismsDestroying protective antibodies
Being enclosed in protective outer coats (capsules Capsules Bacteria are microscopic, single-celled organisms. They are among the earliest known life forms on earth. There are thousands of different kinds of bacteria, and they live in every conceivable... read more ) that prevent defending white blood cells from ingesting the microorganisms (for example, the fungus Cryptococcus Cryptococcosis Cryptococcosis is an infection caused by the fungus Cryptococcus neoformans or Cryptococcus gattii. People may have no symptoms or may have headache and confusion, a cough and... read more
, 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 Biofilm Bacteria are microscopic, single-celled organisms. They are among the earliest known life forms on earth. There are thousands of different kinds of bacteria, and they live in every conceivable... read more , in which they live. The biofilm helps them attach to cells and to foreign material such as intravenous catheters, suture material, prosthetic joints, hardware that keeps broken bones in place (such as pins, screws, rods, and plates), 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 Antibiotic resistance Bacteria are microscopic, single-celled organisms. They are among the earliest known life forms on earth. There are thousands of different kinds of bacteria, and they live in every conceivable... read more ).
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 Overview of Immunodeficiency Disorders Immunodeficiency disorders involve malfunction of the immune system, resulting in infections that develop and recur more frequently, are more severe, and last longer than usual. Immunodeficiency... read more ).
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 , including corticosteroids), such as those used to prevent a transplanted organ from being rejected, or used to reduce inflammation.
