There is substantial data suggesting that keeping your vitamin D levels up will protect you from H1N1 (Swine Flu) and colds and flu in general.
Take a look at these two articles:
August 2009-Vitamin D3 deficiency and its role in influenza
http://archive.constantcontact.com/fs026/1102452079631/archive/1102685428884.html
Sept 2009-More on Vitamin D3 and influenza
http://archive.constantcontact.com/fs026/1102452079631/archive/1102728693089.html
For those of us who are thus far wary of the seasonal flu and/or H1N1 vaccinations, this is something to consider! I am already on a D3 supplement at the recommendation of my doctors. Not only is it safe during pregnancy, it is actually recommended for North Americans, most of whom are severely deficient in D3 and don't even know it! Therefore many babies are born deficient in D3 as well. There may be D3 in your prenatal, but it may be a tiny amount.
My dosage is 5000IU/day but you should definitely talk to your doctor or nutritionist before beginning supplementation. (When my levels were tested I was at a mere 19% of normal-- I never go in the sun without sunblock and haven't for years.)
D3 deficiency is associated will all kinds of issues and is easy to remedy!
Amazing article in Scientific American on this important vitamin (which BTW, is technically a hormone, so it influences the endocrine system as well!)
http://www.scientificamerican.com/article.cfm?id=cell-defenses-and-the-sunshine-vitamin