>Why do people tend to get sick in the wintertime and not the summer?

There are lots of reasons for that. Vitamin-D is just one of those theories. There are also behavioral changes, such as people spending more time indoors and in sealed spaces. There is also the fact that certain viruses may survive better in colder, drier climates. There are also some theories that particles evaporate more quickly in the heat.

Not to mention... humans started wearing closes about 100,000 years ago.