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.
Viral infections include all the child hood illnesses - measles, mumps & chickenpox & also more serious ones such as hepatitis, the Epstein Barr Virus (Glandular Fever) & AIDS/HIV.
Causes
A person with a cold can spread the infection by coughing and/or sneezing.
Bacteria or viruses can be passed on by touching or shaking hands with another person.
Touching food with dirty hands will also allow viruses or bacteria from the intestine to spread.
Body fluids such as blood, saliva and semen can contain the infecting organisms and transmission of such fluids, for example by injection or sexual contact, is important, particularly for viral infections like hepatitis or AIDS.
Viruses can't multiply until they are inside the body's cells. This is the reason why the treatment of virus infections is usually left up to the patient's own immune system, although it may be hard to accept when the doctor says the only cure is for 'nature to take its course'