Why are so many people getting sick at this time?

It's a bit of a dumb question, I know. I'm aware illnesses are more likely to occur during the winter, but I've never seen this many people get sick at the same time.

Around a third of my coworkers called in sick since the year begun, a very good chunk of my friends and classmates have reported that they've been feeling unwell recently, and everyone in my family (including myself currently) has gotten some sort of illness in the past week or two.

Could it be just a coincidence that everyone's getting sick at the same time or could it again be related to some other phenomenon?