There are more than 200 different viruses that cause colds and this is one of the reasons we get so many.
Top of the list are rhinoviruses, but coronaviruses, adenoviruses and respiratory syncytial virus (RSV) can all cause colds, too. So, just as your body develops immunity to one type of virus, another that you haven’t encountered before comes along and makes you ill.
You may have been taught that 'coughs and sneezes spread diseases', but colds aren't usually passed on through airborne droplets or particles.
The most important way they're spread is via direct contact with an infected person. Typically, the person with the cold touches their nose or coughs on to their hand, leaving thousands of microscopic virus particles on their skin.
When they then touch an object such as a door handle, shopping trolley or another human, they leave the virus behind. The next person to touch the object takes the virus with them.
All that's needed to trigger a cold is a dozen or so virus particles. If the person who has picked up the cold virus touches their own nose or eyes, they deposit it in a warm, moist environment where it can thrive.
The virus particles are swept to the adenoids at the back of the throat, where they stick to the cells, invade them and reproduce. Eventually, the cells rupture, releasing thousands of new virus particles to spread to other cells.
This process is remarkably rapid – it takes just eight to 12 hours from the arrival of the virus in your body to release of new virus particles.
Symptoms also develop quickly – within about ten hours (this is known as the incubation period). So, in less than half a day of shaking hands with a person with a cold, you too could have all the unpleasant symptoms.
Symptoms are mostly caused by the body's reaction to the cold virus, which triggers the release of chemicals that make the blood vessels leaky and send the mucous glands into overtime.
As a result, the nasal passages become swollen and choked with secretions.
Symptoms should start to improve after three days and be gone after about seven days (but they may persist for up to two weeks).
As many as one in four people with a cold doesn't develop any symptoms, possibly because their immune system doesn't react to the virus in the usual way.
It's easy to confuse a cold with flu, but in general flu is more severe and produces a higher fever (above 38oC) and more generalised symptoms throughout the body such as aching muscles, lethargy, shivers, nausea and vomiting.
Despite decades of research, there's no simple cure for colds. The antibiotics used against bacterial infections don't have any effect on viruses, and antiviral drugs used for other viral infections aren't useful against the viruses that cause colds.
Most colds resolve rapidly on their own, but there are things you can do to make symptoms less unpleasant.
The benefits of vitamin C supplements for both the prevention and treatment of colds remains controversial.
A recent review of more than 30 studies showed that during periods of intense stress people can half their risk of catching a cold by taking vitamin C. But there's only a very minor effect on the duration and severity of symptoms once someone has the infection.
Some people argue that much higher doses of vitamin C need to be taken.
Many other treatments have been tested. Some, such as zinc and the herbal treatment echinacea, may be useful, but there isn’t enough evidence to recommend these therapies.
If symptoms persist, see your doctor. Serious secondary infections, such as pneumonia, can follow, especially if you have flu rather than a cold. Around one in 20 colds is complicated by a bacterial infection of the sinuses, which needs antibiotic treatment.
Among children, colds often lead to bacterial infections of the middle ear but this is much less common in adults.
Colds can also aggravate asthma and chronic bronchitis.