Viruses are the invading agent responsible for most epidemic illnesses, including colds and influenza, chicken pox, smallpox, poliomyelitis and measles. In addition, a great many vague undiagnosed fevers and many instances of diarrhoea are due to virus infection. Some forms of pneumonia originate with viral infection, while others are bacterial. Most of these are discussed under their individual headings.

A virus is a small infectious organism—much smaller than a fungus or bacterium—that must invade a living cell to reproduce (replicate). The virus attaches to a cell (called the host cell), enters it, and releases its DNA or RNA inside the cell. The virus's DNA or RNA is the genetic material containing the information needed to replicate the virus. The virus's genetic material takes control of the cell and forces it to replicate the virus. The infected cell usually dies because the virus keeps it from performing its normal functions. When it dies, the cell releases new viruses, which go on to infect other cells.

Viruses are too small to be seen by the naked eye. They can't multiply on their own, so they have to invade a 'host' cell and take over its machinery in order to be able to make more virus particles. Viruses consist of genetic materials (DNA or RNA) surrounded by a protective coat of protein. They are capable of latching onto cells and getting inside them. The cells of the mucous membranes, such as those lining the respiratory passages that we breathe through, are particularly open to virus attacks because they are not covered by protective skin.

Some viruses do not kill the cells they infect but instead alter the cell's functions. Sometimes the infected cell loses control over normal cell division and becomes cancerous. Some viruses leave their genetic material in the host cell, where the material remains dormant for an extended time (latent infection). When the cell is disturbed, the virus may begin replicating again and cause disease.

Some viruses (such as rabies, West Nile virus, and several different encephalitis viruses) infect the nervous system. Viral infections also develop in the skin, sometimes resulting in warts or other blemishes 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 . Human herpesvirus 8 has been implicated as a cause of cancer (Kaposi's sarcoma) in people with AIDS. All of the herpesviruses cause lifelong infection because the virus remains within its host cell in a dormant (latent) state. Sometimes the virus reactivates and produces further episodes of disease. Reactivation may occur rapidly or many years after the initial infection.

A few essential oils are powerfully anti-viral, the most important being Bergamot, Eucalyptus and Tea tree. Of these, perhaps Tea tree is the most powerful, although there are few references to it in the standard sources due to its relatively recent introduction to Europe. It also stimulates the body's immune response to the infection.

Baths, vapourisation (and where the respiratory tract is involved, steam inhalation) are the best form of treatment, because there is nearly always fever, and during fever massage is contra-indicated. Vapourisations, whether by means of a burner, electric fragrancer, an aerosol diffuser or such simple means as a few drops on a light bulb or on a wet cloth hung over a radiator, not only help the patient, but are one of the best ways of decreasing the risk of the infection being transmitted to other people in the household.