The diagnosis of a viral infection is usually based on the physical symptoms and the history of the illness. A condition such as influenza, which is caused by a virus, is generally easy to diagnose because most people are familiar with the symptoms. Viral diseases are extremely widespread infections caused by viruses, a type of microorganism. Viral diseases are extremely widespread infections caused by viruses, a type of microorganism. A virus is a tiny organism that causes an infection in the body. Viruses are made up of the genetic material known as DNA or RNA, which the virus uses to replicate. Viruses can affect any part of the body or body system and can cause infections such as the common cold, flu, gastroenteritis, chicken pox or herpes. The most common type of viral infections involves the respiratory tract. The skin can also be infected by a viral infection such as the common wart or chicken pox. It spreads very easily by human contact. Human cells are vulnerable to viruses, and when the body is exposed to viral particles, the immune system will try to destroy these particles and eliminate them from the system. In some cases, viral diseases can lead to serious, possibly life-threatening complications, such as dehydration, bacterial pneumonia, and other secondary bacterial infections. People at risk for complications include those who have a chronic disease or a suppressed or compromised immune system, and the very young and very old. In addition, certain types of sexually transmitted viral infections, such as HIV/AIDS and HPV, can lead to serious complications and death.