A "potentially historic" blizzard is barreling down on us here in New England, and is poised to drop up to two feet of snow on Boston. All of the schools in the area preemptively closed, our public transit system is shutting down at 3:30pm, and trying to buy groceries last night was bedlam. The snow is just now beginning to fall. Winter in New England can be rough, especially for a California-raised boy like me. It's not just because of the snow and cold, it's also the influenza and common colds.
The fact that the rate of some infections can vary by season is not controversial - actually, it was noted by Hippocrates over 2000 years ago - but in many cases, we still don't really know why. In some cases, it's obvious - the bacterium that causes Lyme disease is transmitted by deer ticks, so infections peak in the late spring and early summer when the ticks are active and people are going on hikes. But the winter-peak seasonality of infectious diseases like influenza is harder to pin down.
The first potential explanation is the temperature. After all, there's a disease called the cold for pete's sake. We know that cold temperatures can cause physical stress, and that stress can suppress the immune system. Another potential source of immunosuppression is vitamin D deficiency. Most of our Vitamin D comes from a process called "photoconversion" - ultraviolet radiation from sunlight causes a chemical reaction that produces vitD from 7-dehydrocholesterol. In the winter, especially at northern latitudes, the sun isn't out as long, you're not outside as much and more of your skin is covered up when you are outside. A number of recent studies have suggested many people are vitD deficient during the winter, and that this might have negative impacts on the immune system.
Both of these explanations seem straightforward enough, but neither explain why some pathogens are seasonal and some aren't. If it was just immunosuppression, we would expect all pathogens to be more prevalent or at least more severe in the winter, but this is not the case.
My favorite explanation is the human behavioral one. During the winter, people stay inside more, so they're in closer contact with more people. Infectious disease that spread via direct contact or because of sneezing etc like influenza and the common cold are precisely the pathogens that we'd expect to peak because of this behavior. Unfortunately, this explanation also isn't complete. Why woud there be seasonality in Florida or southern California for this reason? You could argue that people in northern latitudes are the main driver, and act as vectors for more southern populations (actually, I would argue that that's the case).
One of the least satisfying (but still totally valid) potential explanations is that the seasonality occurred by chance. Influenza infections occur in cycles of infection -> immunity -> mutation, where after one round, just about everyone that was susceptible is either immune or dead, and the virus must mutate to become infectious again. It's possible that this annual cycle just happened to peak in the winter. Because of the interactions between different pathogens, this would only have to happen to a single virus in order to get the whole thing going. In other words, the rhinovirus responsible for the common cold* might just be riding the coat-tails of influenza to hit people that are already knocked down by infection. There are issues with this hypothesis too. In the southern hemisphere, flu infection is also seasonal and also falls during the winter, which is off-set by 6 months from the northern hemisphere winter, but this alone does not rule out the "by chance" hypothesis.
There are other possible explanations as well, but all of them are very difficult to test. At best, we have plausible explanations with supporting correlations, but that's not enough to make a definitive claim. Even more problematic, it's possible that all of these explanations are true to some extent, and cooperate to give us the seasonality that we see.
In any case, my fellow East Coasters, bundle up, be safe, and try not to sneeze on anyone.
Most of this information was learned at some point during my schooling, but for a source, check out this review.
*There are actually several different viruses that cause the symptoms associated with "the cold," but the most common is Rhinovirus.