It may be more accurate to speak of flu as infectious, since infection includes contagion. Strictly speaking, a contagious disease is one spread only by direct physical contact, while infection can include any means of spreading including through the air or water. You don’t actually have to touch someone with flu in order to catch it – it is enough to be in the same room, for example, or to touch something that has been handled by an infected person, though of course touching will do the job too – so infection is the more accurate word.
Flu is transmitted in a similar way to a cold, though it is quite a different virus. The full name is influenza; this is the Italian word for influence and reflects an ancient belief that some illnesses were caused by the influence of the planets. Later the word came to be associated with the idea of the ‘influence of the cold’, which isn’t quite accurate either although of course colds and flu are much more common in winter.
As with colds and other viruses, the most typical way to pass on the influenza virus is by coughing and sneezing. This process releases little drops of water into the air, containing the virus, which are then inhaled by other people. Some viruses are passed by direct contact with infected nasal fluids, or by touching objects containing the virus – surfaces, for example, or in some cases bird droppings. Flu is passed on very easily and also mutates a lot – for example when a strain of flu previously found in animals starts to infect humans - making it one of the most infectious of all illnesses. This is why epidemics of flu are so common, and why pandemics – illness on a national or even global scale – have happened throughout history. It seems to spread more easily in cold and wet conditions, partly because there is less sun at such times (sunlight can have the effect of deactivating or weakening the flu virus). It may also spread more easily if people are less careful about hygiene, as detergents and soaps also have a deactivating effect on the virus and washing your hands is one of the best ways to reduce the risk.
As well as being easily transmitted, flu is quite quick-acting. Most people start to develop visible symptoms within one or two days of being infected. It is at this point that the newly infected person is likely to be at their most infectious themselves, as the early symptoms include the coughing and runny noses that help the virus to pass itself on to a new host. Other symptoms include a sore throat (similar to what you get with a cold but worse), headaches, fever alternating with feeling very cold, overall aches and pains and feeling very weak. Some people also vomit when the infection is at its height.
People with a severe or feverish cold often think they have flu, but in fact if you ever do catch flu you will never confuse it with a cold again. Flu really does make you ache all over and in nearly all cases you will have a high temperature, lose your appetite and have to stay in bed. Another unpleasant symptom associated with flu is a feeling of listlessness and depression which often starts a little later.
There are flu vaccines which lessen the symptoms, but there is disagreement about how effective they are, and also if too many are used they may weaken our collective resistance to infection, so they are only recommended for the elderly or otherwise vulnerable people. For most flu patients the best cure is bed rest and plenty of water, though in some cases the illness can be dangerous. You can learn more here.