[cwhiz]

>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.

[tekgnos]

Agree, it is a theory and one I believe. Also how/why white skin evolved is another data point to consider. Lighter skin means more Vitamin D in the summer months which is vital for humans in northern latitudes.

Clothes sure, but still were outside in the sunlight during most of the day without sunblock. Take a look at hunter gather tribes still alive today. Not a whole lot of clothes! But sure, some.