A virus is a small microbiological organism that is able to multiply in living cells and cause disease. Unlike bacteria, viruses are not independent living organisms as they do not have their own metabolism or reproduction. Instead, they require a host cell in order to multiply. Viruses consist of a protein envelope that encloses the viral genome. This genetic material can consist of either DNA or RNA, depending on the type of virus. The viral genome contains the genetic information required for the replication and production of new viruses. Once a virus enters a host cell, it uses its own metabolic processes to replicate. The viral genetic material is integrated into the DNA or RNA of the host cell and takes over its control. The infected cell then begins to produce new virus particles, which in turn can infect other cells. Viruses can cause a variety of diseases in humans, from simple colds to serious infections such as influenza. They can affect the body as a whole as well as attack specific organs or tissues. Viruses are usually spread by droplet infection when infected people cough or sneeze. However, direct contact, for example via blood or body fluids, can also lead to the transmission of viruses. Some viruses can even be transmitted via contaminated water or food.