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:

• 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.

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 Infections: Bacterial Defenses), 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 (the fungus Cryptococcus 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 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 Bacterial Infections: 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 Bacterial Infections: Antibiotic Resistance).

Biological Warfare and Terrorism start
Biological warfare is the use of microbiological agents for hostile purposes. Such use is contrary to international law and has rarely occurred during formal warfare in modern history, despite the extensive preparations and stockpiling of biological agents by most major powers during the 20th century. It is uncertain whether other countries or dissident groups have biologic warfare capability. For a variety of reasons (including uncertain military efficacy and the threat of massive retaliation), experts consider the use of biological agents in formal warfare unlikely. However, biological agents are thought by some people to be an ideal weapon for terrorists. These agents may be delivered clandestinely, and they have delayed effects, allowing the user to remain undetected. Potential biological agents include anthrax, botulinum toxinSome Trade Names BOTOX , brucellosis, hemorrhagic fever viruses (Ebola and Marburg), plague, smallpox, and tularemia. Each of these agents is potentially fatal and, except for anthrax, botulinum toxinSome Trade Names BOTOX , and tularemia, can be passed from person to person. Direct person-to person transmission of brucellosis is extremely rare. Anthrax spores are relatively easy to prepare and, unlike most other agents, can be spread through the air, creating the potential for distribution by airplane. Theoretically, 1 kilogram of anthrax could kill 10,000 people, although technical difficulties with preparing the spores in a sufficiently fine powder would probably limit actual deaths to a fraction of this number. Despite these theoretical concerns, the only successful terrorist use of anthrax—multiple pieces of contaminated mail delivered to a variety of locations in the United States in 2001—resulted in only a handful of deaths and a small number of serious infections (22 total cases). More people were contaminated with anthrax spores without developing illness, possibly because of extensive use of the antibiotic ciprofloxacinSome Trade Names CILOXAN CIPRO . However, there was extreme public anxiety related to these incidents. The number of false threats of anthrax reported has been very large. In 1999, the FBI received an average of one false report of anthrax use per day. Even more false reports, both hoaxes and reports by alarmed citizens mistaking harmless material for anthrax, were reported after the 2001 anthrax attack. The only other successful use of a biological agent by a terror group in the United States occurred in 1984. In this event, 751 people developed diarrhea resulting from the intentional contamination of a salad bar with Salmonella in Oregon. The bacteria were introduced by a religious cult trying to influence the results of a local election. No one died, and the election was not affected. Defense against bioterrorism involves several factors: Intelligence information to disrupt the terrorists before they can use the weapons Early detection Availability of protective antibiotics Immunization of selected populations (such as the military)

Infection From Medical Devices
Usually, people think of infection as occurring when microorganisms invade the body and adhere to specific cells. But 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.

The immune system may not function well because

• People are born with a hereditary disorder (an immunodeficiency disorder) that impairs it.
• 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 (such as those used to prevent a transplanted organ from being rejected or corticosteroids, used to reduce inflammation—see Immunodeficiency Disorders).

Last full review/revision October 2012 by Allan R. Tunkel, MD, PhD