One of the issues is lack of vitamin D.

The reason why we've evolved to store vitamin D in fat is similar to the reason why hibernating animals such as bears store energy in fat for the winter. Sunlight. There's just not very much of it during the winter months. Vitamin D interacts positively with the nucleus of white blood cells called T-Cells, triggering them to start multiplying, which are one of the immune system's first lines of defense.

With people working in doors, living in doors, driving in their cars, riding in buses, trains, etc. we're getting less and less sunshine than we used to and when winter comes, our immune systems' are less equipped to handle flu season.

The quickest fix... as if there is a quick fix, is to take a 5,000 iu - 10,000 iu vitamin D supplement each day during the winter months. the dosage depends on how dark your skin pigmentation is (darker pigmentation = less vitamin D absorption from sun) and the latitude in which you live, and the amount of time you get out in the sun.

For reference, 10,000 iu vitamin D is roughly equal to the amount a light skinned person would gain from being in sunlight in their swimsuit for an hour.

NOTE: For health reasons, if you take higher dosages (which are safe), individuals should also take magnesium citrate to counter the higher levels of calcium absorption that comes with high does vitamin D. High levels of calcium absorption can lead to muscle cramps and heart problems. Also, it's important to note that higher ng/ml of vitamin D levels reduces heart disease over all, in addition to higher magnesium citrate supplementation.

Additional reading.
http://www.forever29nutrition.com/lower-cancer-risk-83/

http://www.forever29nutrition.com/gisele-bundchen-thinks-sunscreen-sucks/

http://www.forever29nutrition.com/heart-disease-avoid-1-thing-do-1-thing/