Why do people catch the flu in the winter? The answer to the riddle comes to us from the unlikeliest of places: a maximum security prison!

Back in 2005, the Atascadero prison in California was hit with a severe flu outbreak. Inmates from all over the prison got sick. Yet one ward was unaffected. None of the prisoners in that ward got the flu - even after they mingled with infected inmates from other wards!

Why didn't the prisoners in that ward get sick? It turns out that the doctor in that ward was doing something the other doctors weren't: He was giving the prisoners daily doses of vitamin D.

Yes, vitamin D. We've known for years that vitamin D protects you against a whole host of illnesses, including osteoporosis and cancer. But recent research shows that vitamin D protects you against infections, too. That's because vitamin D stimulates your body to make a powerful germ-fighting substance called cathelicidin. And some scientists believe that cathelicidin just might be the most powerful natural antibiotic ever discovered!

It all makes sense. We know that our bodies make less vitamin D in the winter because there's less sunlight. So the reason we get sick in the winter has nothing to do with the cold; it has to do with the fact that our vitamin D levels are lower!