A virus infection is an infective agent smaller than a bacterium, is not a cell, is unable to be seen using a light microscope, has no internal metabolic processes, and is unable to replicate without the use of a living cell.
Viruses are unimaginably small, and millions could exist on this full stop. They can be found anywhere in the environment – in the body, or in a drop of sweat, in saliva, or the skin of the family dog.
In the body, they will be under constant attack by the body’s defence system. Every minute, millions more viruses enter the body through the mouth or nose. As they enter, the defence system uses its special cells and protein particles (antibodies) to repel the attack. Sometimes the defences are overwhelmed for a short time by the rapidly multiplying viruses. When this happens, the patient may feel off-colour for a day or two. If the virus numbers manage to totally defeat the defenders, a full-blown viral infection will develop. Viruses can cause diseases as diverse as measles, hepatitis, cold sores, chickenpox, glandular fever, AIDS and the common cold.