Viruses, unlike other microorganisms such as bacteria, cannot reproduce independently. A virus has to enter a living organism and take control of some host cells in order to make more copies of itself. Viruses are very specific in their choice of host. There is little cross infection between animals and humans. The majority of viruses are well adapted to their host (i.e. you) and usually cause a self limiting illness.

A virus enters your body by being swallowed with food or drink, licked off fingers, inhaled in an aerosol of tiny virus particles from somebody's sneeze, transferred skin to skin, injected into the bloodstream or transmitted sexually. Whichever route is a particular virus's favourite, it will only attach itself to certain cells in your body that carry a protein on their surface that the virus recognises as a "receptor" for itself. Once attached to the receptor on a cell the virus inserts its nucleic acid (its genetic code), and sometimes a few proteins that help it reproduce.

Viruses are dependent on your own body processes to reproduce. The virus's genetic code is not dissimilar to human nucleic acid (your genes) and the virus uses your body's own processes to make more copies of the virus genetic code and proteins. These are assembled into virus particles and go on to infect other cells or people. When a virus makes more copies of itself it may or may not kill the cell it is using. This depends on the type of virus.

Your body can recognise when it is infected by a virus. The virus leaves tell tale signs that your immune system can detect. Your immune system responds by producing more immune system cells and a variety of chemicals. In combination these stop your body making more virus and kill virus infected cells. The body also produces antibodies that prevent virus spread within your body and give lasting immunity from further infection by that strain of virus.

Unfortunately, all this immune system activity is responsible for the 'flu symptoms that are common with viral infections. Fever, localised pain, headache, muscle and joint aches all contribute to make you feel miserable. Paracetamol will help to minimise these. Console yourself that your flu-like symptoms are there because you are mounting an efficient response to the virus.

Because viruses depend on your own body processes to reproduce, it is difficult to design drugs that inhibit this process without simultaneously damaging your cells. Regrettably antibiotics used to treat bacterial infections will not be the slightest use in stopping a virus infection. There are just a handful of antiviral treatments for particular infections only. For the vast majority of viruses that you will catch there is no treatment that the doctor can offer you to speed recovery. Fortunately most viral infections are self limiting-the body is efficient at clearing the infection within a few days.

Typical infections that are due to viruses include the common cold. Any illness characterised by fever and a runny nose whether or not this is accompanied by sore throat, headache, muscle and joint pains, is probably due to a virus. There are thousands of strains of viruses that cause these symptoms and it would be impossible to become immune to all of them. The treatment here is paracetamol and time.

Most sore throats are caused by viruses and will not respond to antibiotics. Most chest infections described as "flu" are due to viruses such as influenza and parainfluenza. Again antibiotics will not help. Usually these are self limiting illnesses and will settle, although the residual cough often continues for several weeks after the virus infection is controlled. Paracetamol will help to control the 'flu symptoms.

Childhood diarrhoea and vomiting is usually due to a viral infection. The treatment is adequate fluid replacement using a balanced electrolyte replacement such as Dioralyte if the diarrhoea is profuse. Diarrhoea may continue for a week or more but your child can eat normally.