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.
Antibiotics are drugs used to treat bacterial infections. They are not effective against viral infections and most other infections. Antibiotics either kill bacteria or stop them from reproducing, allowing the body's natural defenses to eliminate them.
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 are classified by how they appear under the microscope and by other features. 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. Gram-positive bacteria stain blue. 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.)
Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and is treated with antiretroviral medications. If untreated, it can cause acquired immunodeficiency syndrome (AIDS).
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.
Sexually transmitted infection (STI) refers to an infection that is passed through blood, semen, vaginal fluids, or other body fluids during oral, anal, or genital sex with an infected partner. Sexually transmitted disease (STD) refers to a disease that has developed from an STI.
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.