There's an hypothesis, which conceptually makes sense though not sure how proven it is, about why flu season is Jan/Feb - March/April.

Basically during Fall as the amount of sunlight drops our bodies vitamin D, for which there is clinical evidence that it helps with immunity, drops leaving us more susceptible to infections leading to the overall flu season starting in Jan/Feb time frame. As the sunlight picks up, vitamin D increases and flu season tapers off.

Do you have more info about that hypothesis? I always thought that the usual explanation was simply that people are inside buildings and cars (and thus around other people) more often when it is colder outside.

I'd love to know the answer to this, if there is one.

Anecdotally: I have seen a couple studies pop up recently talking about how daily value of vitamin D has been wrong for a long time. This makes me think taking vitamin D might be the new fad medical thing to do, kinda like how going gluten free was the big thing a couple years ago.

For transparency I have not read all of these but they all have popped up recently: https://news.ycombinator.com/item?id=15867918 https://news.ycombinator.com/item?id=16062697 https://news.ycombinator.com/item?id=15877289 https://news.ycombinator.com/item?id=15687217

Except that in this case you can just go to your doctor and get your vitamin D level tested. Then you adjust your dosage as needed to reach the recommended blood concentration. Adequate vitamin D levels are the subject of a little controversy [0], but I don't think it is a fad to consider that people need a certain level of vitamin D.

[0]https://www.health.harvard.edu/blog/vitamin-d-whats-right-le...

Alternative thoughts on adequate vitamin D: https://www.vitamindcouncil.org/i-tested-my-vitamin-d-level-...

There have also been randomized controlled trials of Vitamin D vs placebo measuring flu rates. This is one meta-analysis I found: http://www.bmj.com/content/356/bmj.i6583

There was a study linked on HN related to an error in Vitamin D dosage recommendations. Original article:

https://www.ncbi.nlm.nih.gov/m/pubmed/28768407/

Vitamin D is relevant to proper functioning of immune system, however it seems that it is hard to find "the right amount" for everyone as it depends on several factors.

I'm wondering if Vitamin D intake can improve response against recurring infections like flu, sometimes they can bring you down several times in a season, so it takes quite a heavy toll on productivity and mood.

As I understand it - many people in the northern hemisphere are Vitamin D deficient and that can lead to suppressed immune function.