Bless you! That sneezing, sniffing and coughing can mean only one thing - you've caught a cold or, worse still, the flu. And this computer-generated image shows how it happens.
Spiky viruses (like the one on the left) fly down your throat or up your nose before landing on a cell (the bottom part of the picture). Then they inject genetic material, hijacking the cell's reproductive system and ordering it to make more viruses (those in black cocoons), which are then released to spread the infection.
Viruses are the chameleons of the medical world, constantly mutating into new forms, and taking a pop at our bodies' defences. Their changeable nature means that we are still no nearer finding a cure for the common cold, which wears more than 200 disguises.
There are only three main types of flu virus - A, B and C - but many variations, depending on the combination of proteins (15 types of haemagglutinin and nine of neuraminidases) found in the virus's coat.
Type C can be so mild it's mistaken for a cold whereas Type A can be deadly - the AH1N1 strain caused the Asian flu epidemic of 1918-19 which killed more than 20 million people and AH3N2 is behind this winter's high incidence of flu.
But, despite the scare stories, it isn't an epidemic. Four hundred out of every 100,000 people have to become infected every week for it to qualify for that title according to the World Health Organisation, which monitors outbreaks of flu around the world and develops vaccines to nip them in the bud.
Viruses can also carry anything from warts and mumps to rabies and Aids. But their bad guy reputation could be on the line. Scientists have found a way of harnessing viruses' contagious qualities to ferry healthy genes into cancerous cells. So, in the future, these microscopic space invaders could be on our side.