Just A Cold, Bro

In the "age" of coronavirus, Wikipedia's common cold page is enlightening. https://en.wikipedia.org/w/index.php?title=Common_cold&oldid=1132786469

"Well over 200 virus strains are implicated in causing the common cold, with rhinoviruses, coronaviruses, adenoviruses and enteroviruses being the most common."

haha, oops

"There is no vaccine for the common cold." haha, oops! There continues to be no vaccine.

"The primary methods of prevention are hand washing; not touching the eyes, nose or mouth with unwashed hands; and staying away from sick people." 

Naturally the real method of prevention is vitamin D. It's literally a steroid - it makes your white blood cells hueg instead of your muscles. Does a bunch of other cool stuff too, but that's the noticeable bit. 

"Under normal circumstances, the average adult gets two to three colds a year, while the average child may get six to eight."

Oof! I considered it an especially bad year when I got two. I can tell I'm taking enough D when it's zero. 100% less is a risk factor of infinity...

"There is no conclusive evidence that vitamin D supplementation is efficacious in the prevention or treatment of respiratory tract infections"

Milady doth protest too much. They have to specifically address it because it works, and they hate it when you're too healthy. (See also: meat-eating recommendations.)

To some extent this isn't sheer dishonesty. The correct dose of vitamin D is twice or more the "safe upper limit" so they're not allowed to administer it under normal circumstances. ("Doses used in the constituent studies that used daily dosing ranged from 300 to 4000 IU/day." 300 is literally nothing.) Note that going to the beach for an hour in summer can give you up to five times the "safe" limit. Evolution: smarter than you. That and every nutritionist in the world is worse than sewage, which is where the "safe" limit comes from.

There may also be important co-factors, but I've never been deficient in them, so I wouldn't know.