|MadSci Network: Medicine|
Common colds are caused by viruses that infect the "upper respiratory tract," which pretty much means the sinuses. The sinuses get irritated and produce lots of mucus — the stuff that runs out of your nose or gets sneezed out. Some of the mucus drains down the back way into your throat, and often comes out in coughs. This mucus with the viruses in it is the main way that colds are transmitted — through the air inhaled into someone else's nose. However, the mucus that drained into the throat will get mixed with saliva in the mouth to some extent. There's no way to keep viruses just in the back of the throat. So, there's a little bit of virus in the saliva, and a little bit of the saliva gets in or on the shared drink. Swallowing the cold virus is a much less efficient way than inhaling the virus to catch a cold, but I wouldn't chance it. Or, I'd have to be really thirsty!
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