Viruses are capsules with genetic material inside. They are very tiny, much smaller than bacteria. Viruses cause familiar infectious diseases such as the common cold, flu and warts. They also cause severe illnesses such as AIDS, smallpox and hemorrhagic fevers.

Viruses are like hijackers. They invade living, normal cells and use those cells to multiply and produce other viruses like themselves. This eventually kills the cells, which can make you sick.

Viral infections are hard to treat because viruses live inside your body's cells. They are "protected" from medicines, which usually move through your bloodstream. Antibiotics do not work for viral infections. There are a few antiviral medicines available. Vaccines can help prevent you from getting many viral diseases.

Types of Viral Infections:  Probably the most common viral infections are those of the nose, throat, and upper airways (upper respiratory infections). These infections include sore throat, sinusitis, and the common cold. Influenza is a viral respiratory infection. In small children, viruses also commonly cause croup and inflammation of the windpipe (laryngotracheobronchitis) or other airways deeper inside the lungs (bronchiolitis-see Respiratory Disorders: Bronchiolitis). Respiratory infections are more likely to cause severe symptoms in infants, older people, and people with a lung or heart disorder. 

Some viruses (such as rabies, West Nile virus, and several different encephalitis viruses) infect the nervous system (see Infections of the Brain and Spinal Cord: Viral Infections). Viral infections also develop in the skin, sometimes resulting in warts or other blemishes (see Viral Skin Infections: Warts). 

Other common viral infections are caused by herpesviruses. Eight different herpesviruses infect people. Three of them-herpes simplex virus type 1, herpes simplex virus type 2, and varicella-zoster virus-cause infections that produce blisters on the skin or mucus membranes. Another herpesvirus, Epstein-Barr virus, causes infectious mononucleosis. Cytomegalovirus is a cause of serious infections in newborns and in people with a weakened immune system. It can also produce symptoms similar to infectious mononucleosis in people with a healthy immune system. Human herpesviruses 6 and 7 cause a childhood infection called roseola infantum (see Viral Infections: Roseola Infantum). Human herpesvirus 8 has been implicated as a cause of cancer (Kaposi's sarcoma) in people with AIDS.