A natural flu killer? Maybe
Influenza outbreaks run in seasonal cycles, the US right now experiencing a cold weather swine flu epidemic, President Obama declaring the outbreak a "national emergency". The reasons for this seasonality are not well understood. One elegantly simple explanation is that people are most susceptible to infection over winter or during the rainy season when their vitamin D levels are lowest – vitamin D is produced by the body on exposure to sunlight. It is thought that lowered vitamin D levels sufficiently weaken the immune system that the body is unable to fight off the flu.
It might therefore be a good idea to take a vitamin D supplement (D3 or cholecalciferol), especially during periods of low sun exposure. Vitamin D is relatively inexpensive but large doses can be toxic – not nearly as toxic as vitamin A, but toxic nonetheless (see here). The vitamin might have other benefits as well, Dr Soram Khalsa noting that a long term deficiency is "associated with 17 types of cancer, heart disease, high blood pressure, osteoporosis, autoimmune diseases including multiple sclerosis, as well as chronic pain and a wealth of other modern chronic diseases." (Go here for more info including suggested dosage.)
Note: I have no medical or science qualifications. Do not have a big cry in my direction if you take vitamin D and get the flu or if you poison yourself with huge doses.