• Viruses exist through symbiosis - an intimate association between two forms of life.
• Viruses can only reproduce inside living cells where they turn the host's cells into virus-making copying machines.
• Once the viral information becomes integrated into a host's genome, it becomes part of the host and passes on for generations.
• In most cases, viruses don't cause disease, and many are beneficial. Viruses can prevent bacteria from invading our gut to cause infections.

There are three dominant models of viruses that seem to complete each other.

• The "virus first" model suggests that simple forms of viruses existed before cells and may have provided the raw material for the development of cellular life.
• "Escape" or progressive model proposes that viruses come from genetic elements that escaped from the genes of larger organisms.
• The "reduction" model suggests that viruses were once larger, free-living organisms that lost their genetic information and ended up smaller and unable to reproduce except inside the cells of other organisms.

In 1992, the giant virus, or mimivirus, was first observed during the pneumonia outbreak in England. Later the mamavirus and the even larger pandoravirus stretched the human perception of what it means to be a virus.

Giant viruses are not known to affect human health dramatically. The Asfivirus - the African swine fever virus - transmits from tick to pigs and causes hemorrhagic fever. It is closely related to a giant virus from the sewage in Marseille.

In intact ecosystems, like healthy forests, viruses and their wild hosts interact in a delicate balance. But when humans interfere, they can expose themselves to unknown viruses.

When humans modify landscapes, it encourages interactions between people and wildlife and can affect infectious disease emergence. Forest degradation has been associated with outbreaks of viruses that originated in wildlife, such as Nipah, Hendra, and the Ebola virus.