Bacteria, fungi, viruses, and protozoa are microorganisms. They live everywhere in the world, and some naturally live in the human body. People and microorganisms generally co-exist peacefully, often in mutually beneficial ways. But, if a microorganism invades a part of the body where it is not welcome, or multiplies beyond its usual number in the body, it can cause an infection.

Viruses are much smaller than bacteria or fungi and cannot be seen with a light microscope. A virus generally lives inside of another cell, such as a human cell, and releases its DNA and RNA (its own genetic code) in the process of replicating or multiplying itself. A virus can affect the host cell’s functioning and perhaps even kill the cell it has infected. Most viral infections are transmitted from one person to another. Well-known examples include viruses such as rhinoviruses (the common cold), flu viruses (influenza), rubella and rubeola (measles), herpes, human papillomavirus (which causes genital warts), viruses which cause liver inflammation (hepatitis), and HIV.

In the past, when diagnosing a viral infection, doctors would screen for the development of antibodies in a patient’s blood (serologic diagnosis) against the virus. Unfortunately, serologic diagnoses are often not useful, since a patient may be well into the infection or recovered from it by the time the antibody test turns positive.

New methods to rapidly identify viruses have recently been developed, and although these methods are not usually as sensitive as tissue culture techniques, which involve actually growing the virus on cells, they are much faster and easier to do. Culturing viruses is an expensive, specialized process that involves keeping a supply of living cells to inoculate (tissue culture). Some viruses are difficult to grow, even when tissue culture is available. Vaccines exist to prevent many viral infections, but others remain elusive, and effective antiviral drugs are often difficult to design.

As noted, HIV infection, herpes, hepatitis, and genital warts are all caused by viruses.