It's that time of year - the sniffling, sneezing, and all out painful head in the clouds season. Though the flu has been a hot topic and the common cold is often blamed for these ailments, there are literally hundreds of infections that could be behind what any one person has at a given time.

A cough is a reflex to clear the throat and lower respiratory tract of irritants. In the same way, a sneeze is a physiological mechanism to clear the upper respiratory tract and nasal passages. It can be due to simple dry irritation of the mucus membrane in the throat, a respiratory infection anywhere in the throat or lungs, bronchial inflammation, asthma, allergy, etc ad nauseum. Anything that irritates the throat or lungs will produce a cough, anything that irritates the nose will produce a sneeze.

This includes the mucus production caused by cold and flu, which aren't the same thing, though we tend to lump them together.

Both colds and the flu are caused by viruses. (Usually, there are other types of infection, like Haemophilus influenzae (the stomach flu), that are called flu, but aren't actually influenza). The flu virus infects the lower respiratory system, the cold virus infects the upper respiratory system (usually, the little buggers don't always follow directions). Influenza occurs as A, B, or C virus, and A viruses are subtyped based on at least two surface proteins (H and N). The various types infect different species, including humans, birds, pigs, cats, dogs, horses.Influenza A is the main culprit behind the Winter scourge, but the strain circulating each year is pretty constant. So we vaccinate.

Colds, on the other hand, are caused by more than 200 different viruses. Approximately half of all colds are thought to be caused by rhinovirus, but there are 100 different serotypes of rhinovirus. So your cold is likely caused by one of a hundred rhinoviruses from three different genetic classes, or adenovirus, or a number of other viruses. So we treat the symptoms.

Colds are usually self-limiting, though recent research has found associations with later asthma and infection with a novel class of rhinovirus. Influenza can be life-threatening, though it is usually self-limiting in healthy individuals. They both have similar symptoms, but the flu brings fever.

So please, keep your pathogen party to yourself. The next time you cough or sneeze, cover your mouth and wash your hands, you're spreading 100 million viral particles*, whether you feel it or not. 

*This is not an exaggeration. Flint et al. Principles of Virology: Molecular Biology, Pathogenesis, and Control. ASM Press, 2000.