Infections

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 of microorganisms invade, multiply, and cause disease in people.

Infectious diseases are caused by microorganisms, such as bacteria, viruses, fungi, and parasites.

Antibiotics are drugs used to treat bacterial infections. They are ineffective against viral infections and most other infections. Antibiotics either kill microorganisms or stop them from reproducing, allowing the body's natural defenses to eliminate them.

Arbovirus, arenavirus, and filovirus are viruses that are spread from animals to people and, with some viruses, from people to people. The animal involved depends on the type of virus.

Bacteremia, sepsis, severe sepsis, and septic shock are related:

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 environment all over the world. They live in soil, seawater, and deep within the earth’s crust. Some bacteria have been reported even to live in radioactive waste. Many bacteria live on and in the bodies of people and animals—on the skin and in the airways, mouth, and digestive, reproductive, and urinary tracts—without causing any harm. Such bacteria are called resident flora, or the microbiome. There are at least as many bacteria in our resident flora as there are cells in the body. Many resident flora are actually helpful to people—for example by helping them digest food or by preventing the growth of other, more dangerous bacteria.

Bacteria can be classified in several different ways. One way is based on their need for oxygen—whether they need oxygen to live and grow:

Gram-negative bacteria are classified by the color they turn after a chemical process called Gram staining is used on them. Gram-negative bacteria stain red when this process is used. Other bacteria stain blue. They are called gram-positive bacteria. Gram-negative and gram-positive bacteria stain differently because their cell walls are different. They also cause different types of infections, and different types of antibiotics are effective against them.

Gram-positive bacteria are classified by the color they turn after a chemical called Gram stain is applied to them. Gram-positive bacteria stain blue when this stain is applied to them. Other bacteria stain red. They are called gram-negative. Gram-positive and gram-negative bacteria stain differently because their cell walls are different. They also cause different types of infections, and different types of antibiotics are effective against them.

Bejel, yaws (frambesia), and pinta are infections caused by Treponema, which are spiral-shaped bacteria called spirochetes (see figure How Bacteria Shape Up).

Chlamydia are bacteria that cause disease in humans, including sexually transmitted diseases (STDs), and infections of the eyes and respiratory tract.

Enterovirus infections affect many parts of the body and may be caused by any of several different strains of enterovirus.

Fungi are neither plants nor animals. They were once thought to be plants but are now classified as their own kingdom. Some fungi cause infections in people.

Some common viral infections are caused by herpesviruses. Eight different herpesviruses infect people:

Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and can cause acquired immunodeficiency syndrome (AIDS).

Immunization enables the body to better defend itself against diseases caused by certain bacteria or viruses. Immunity (the ability of the body to defend itself against diseases caused by certain bacteria or viruses) may occur naturally (when people are exposed to bacteria or viruses), or doctors may provide it through vaccination. When people are immunized against a disease, they usually do not get the disease or get only a mild form of the disease. However, because no vaccine is 100% effective, some people who have been immunized still may get the disease.

Meningococcal infections are caused by the bacteria Neisseria meningitidis (meningococci) and include meningitis and sepsis.

A parasite is an organism that lives on or inside another organism (the host) and benefits (for example, by getting nutrients) from the host at the host's expense. Although this definition actually applies to many microbes, including bacteria, fungi, and viruses, doctors use the term "parasites" to refer to

Echinococcosis is caused by the dog tapeworms Echinococcus granulosus and Echinococcus multilocularis. In people, the tapeworms can cause fluid-filled cysts or masses to form in the liver or other organs.

African sleeping sickness is an infection caused by the protozoa Trypanosoma brucei gambiense or Trypanosoma brucei rhodesiense. It is transmitted by the bite of a tsetse fly.

Amebiasis is an infection of the large intestine and sometimes the liver and other organs that is caused by the single-celled protozoan parasite Entamoeba histolytica, an ameba.

Ascariasis is infection caused by Ascaris lumbricoides, an intestinal roundworm, or occasionally by Ascaris suum (which causes ascariasis in pigs).

Flukes are parasitic flatworms. There are many species of fluke. Different species tend to infect different parts of the body.

Monkeypox is caused by the monkeypox virus, which is related to the smallpox virus and causes a similar, but usually milder illness.

Bird flu is a viral infection with strains of influenza virus that normally occur in wild birds and domestic poultry.

Rickettsial infections and related infections (such as anaplasmosis, ehrlichiosis, and Q fever) are caused by an unusual type of bacteria that can live only inside the cells of another organism.

Sexually transmitted (venereal) diseases are infections that are typically, but not exclusively, passed from person to person through sexual contact.

Tuberculosis is a chronic contagious infection caused by the airborne bacteria Mycobacterium tuberculosis. It usually affects the lungs.

A virus is composed of nucleic acid, either DNA or RNA, surrounded by a protein coat. It requires a living cell in which to multiply. A viral infection can lead to a spectrum of symptoms from asymptomatic (no overt symptoms) to severe disease.