I think most of us wash our hands about the same amount in summer and winter. But we're much more likely to get sick in the winter, especially from colds and flu. So all that hand sanitizer and all the advice to “Wash your hands, wash your hands, wash your hands” does not appear to be the whole story.
What's different in summer? Well, one obvious factor is more exposure to sunlight. And all that advice about “sunscreen, sunscreen, sunscreen” is not the whole story either. Sunlight converts cholesterol into Vitamin D. Northern Europeans evolved skin with very little pigment because in the days before dietary supplements, they needed all the sunlight they could absorb. Dr. Sharon Moalem points out in his best-selling book that the Innuit people of the subarctic have relatively dark skin even though they get minimal sunlight, but their diet contains large quantities of fatty fish, rich in Vitamin D. Europeans without this dietary advantage had to lose their pigment to survive.
Dr. Moalem also notes that northern Europeans, with little sunlight, and sub-Saharan Africans, with dark skin, are both at increased risk for high cholesterol. He concludes that people at risk for Vitamin D deficiency evolved high cholesterol levels to increase the production of the vitamin from sun exposure.
Vitamin D is traditionally known for its effects on bone growth. But over the last 25 years, the role of this vitamin in the immune system has come into focus. The innate immune response is our first line of defense against infection, and is particularly important in fighting germs that mutate quickly, such as the viruses that cause colds and flu. Vitamin D boosts the production of antimicrobial peptides in this response.
The adaptive immune response is a slower but more powerful process. When something goes wrong, this is the source of autoimmune disease. Vitamin D plays a role in regulating adaptive immunity. Vitamin D deficiency is associated with type 1 diabetes mellitus, multiple sclerosis, Crohn's disease, and asthma.