A virus is a particle of infectious material. Viruses are incredibly small and are only visible with extremely strong microscopes. When visualizing the size of a virus, some people use the analogy of a human standing next to Mount Everest, with the human being the virus and the mountain being a flea. Many people are familiar with viruses because they cause disease as part of their life cycle. Scientists who study viruses are known as virologists; virologists attempt to identify and isolate viruses in the hopes of being able to treat them or vaccinate against them.
The existence of viruses began to be hypothesized as early as the 1800s, although scientists did not positively identify them until the 1900s. The word is taken from the Latin virus, which refers to a toxin or a poison. Some well known examples of viruses are Ebola, HIV, influenza, and Marburg. Many of these viruses are famous for their virulence and they are notoriously difficult to treat since they mutate rapidly and very effectively.
The particles consist solely of a protein covering which encapsulates genetic material. Viruses are incapable of reproducing or living on their own; they require hosts to survive and pass on their genes. As a result, many scientists are reluctant to classify viruses as living organisms. The deceptively simple particles also behave very differently from other forms of life, making them difficult to classify.
When a virus enters a host, it latches on to a cell in the host's body. The cell is essentially hijacked and forced to reproduce the genetic material of the virus. In some cases, the virus may remain latent, but when it is stimulated, the cell will burst open, flooding the host's body with many copies of the virus which can go on to colonize other cells. The original host cell dies, which can be a serious problem for one celled organisms like bacteria; a virus which preys on bacteria is called a bacteriophage.
Treating viruses is extremely difficult. Since they are not alive, drugs like antibiotics are not effective. Antiviral drugs usually focus on attacking the proteins in the virus, in the hopes of crippling it so that it cannot continue colonizing the host. The best cure for a virus is actually a vaccine, because vaccines will prevent colonization in the first place, by teaching the cells of the host to attack viral particles.