As Karla mentioned in her post on Cold and Flu Season, vitamin D can play a role in preventing a more severe case of the seasonal flu. In addition sufficient levels of vitamin D are known to help prevent high blood pressure, certain cancers, osteoporosis, kidney disease, liver disease, multiple sclerosis and more.
Studies have shown that as many as 60% of us are deficient in vitamin D. Some common reasons for vitamin D deficiency are being overweight (vitamin D is fat soluable, it can be taken into fat cells and stored, thus making it potentially less available in our body's metabolism.), being dark skinned (it takes more sunlight for your body to absorb the vitamin D) or simply due to your geographic area (you simply are not exposed to as much sun).
Sun is the best source of vitamin D. In the U.S. if you live north of Texas you are probably not getting sufficient amounts of sunlight starting in the Fall. Light skinned individuals require 10 - 15 minutes of sunlight 2-3 times per week when the shadow you cast is shorter than you. In other words, the sun is high in the sky. Dark skinned individuals require one hour.
Fish such as salmon, shrimp and cod are a excellent to very good sources of vitamin D. If you are not likely to eat the amount of fish required to maintain a good vitamin D level, you may supplement with cod liver oil.
Dairy products are often fortified with plant based vitamin D. These can be good sources of vitamin D, but are not as good for humans as the animal version.
Eggs are another good source of vitamin D.
The only way to really know your vitamin D levels is to be tested. Your doctor can then interpret your vitamin D level and recommend dietary changes or supplements if necessary.