Can you have two colds at once?

Can you get two colds at once? If not why not?

• Answer:

  You can get two different viral infections at the same time, but there are some interesting interactions that make dual infections less likely (apart from the raw probability issue - if you get infected with 10 viruses per year, the odds of getting two on the same day are, hmm, 2.1% if I did that right, and I may not have). The confounding factor is innate immunity. We have, very broadly speaking, two forms of immune response - one that is triggered by a wide range of pathogen-associated molecules and that turns on a broad-based anti-pathogen response, and a second form (acquired immunity) that is turned on by a very specific pathogen molecule and that very precisely targets a piece of a pathogen. If you are infected with virus A today, your innate immune response will turn on in minutes, and will ramp up for a couple of days.  If you're infected with virus B a month later, the innate response will turn on, pretty much at the same rate, and ramp up at the same rate.  (If you were infected with virus A for a second time, then your acquired immune response would rapidly turn on and hopefully eliminate virus A, while virus B wouldn't interest the acquired response until it came around for the second time itself.) If you're infected with virus A today, and virus B in two days, then B will be entering your body at around the peak of the innate response to virus A.  But the innate response doesn't much care about the details; it is anti-viral, not anti-virus A.  So virus B will have a much harder time establishing an infection than if it were to arrive in a month, after the innate response shuts down. So you can have two colds at once, but it is a harder challenge for the viruses.  Doesn't mean it doesn't happen, but it's probably less common than the probabilities would suggest.