system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do seasonal changes in weather make everyone sick?

I used to work in a pharmacy and every significant change in weather opened the floodgates of sickness: coughs, colds, runny noses, etc. 

Why?

Also, what's the best thing to do to prevent or stop it?
gpt
Certain viruses and bacteria spread and multiply better in different conditions. People haven't been exposed to these diseases in months and thus may not have a high level of resistance, although they will likely build up a resistance by the end of the season.

Also, changing weather results in changes in human activity. People are more likely to go outside when it gets warm out, where they're exposed to pollen and other allergens (seasonal allergies are often mistaken for colds). When it gets cold out, people tend to stay inside in close contact with everyone else, which allows for easier spread of diseases.

Another point is that heating and cooling systems are turned on, and if they haven't been cleaned they can have buildups of allergens like dust or mold growing in them.

As far as prevention goes, the most reliable advice would be to practice good hand washing (including before meals whenever possible), get plenty of sleep (as a lack of sleep can weaken the immune system), and take allergy medication if you get any seasonal allergies.