If you work in an office, you know what time of year it is. It's when sounds of coughing and sneezing mix in with the usual office chatter. And maybe when you wish your sniffling, hacking co-workers would just stay home already.
The conventional wisdom goes that those people who still come in to work when they're sick are putting their healthy colleagues at risk. Or that they're not actually being very productive anyway, and would do better to stay home and rest up.