The immune system has memory, the only way...at all..ever...it can be effective is against virus's known to it.
Hence, flu jabs, though that is good guess work, due to the flue strains predictions can be made to how the flu virus evolves, theirs a "trend" you get a "small" infecting when you get a jab..the immune system picks up in it...works with that...and knows what to look for should you pick it up in the wild.. It basically how ever single vaccination you have ever had as a kid works.
Until the super flu comes along that will wipe out a few million...and that's been talked about a lot..yea...we can all look forward to that one day. Just to give you another worry in your life lol.
So, if the immune system has no memory of what to look for when a new virus enters the battery..it happily ignores it for a short time, by that time you face deep in sinus gung not wanting to leave your bed and left with a strong desire to stay in bed with tea and terrible day time television for a couple of days.
Yes., you can at time but a lot of stress on the immune system, terrible rest in general, stress/ anxiety, though not much.
Coming off the back of chemo, or during an influenza infection ( was hospitalised with that myself, took about 6 months to get back to normal, also got shingles as my system couldn't defend myself)
Does taking all those supps mean you are better defended against a flu or cold virus..or any other kind of season bug...no..not at all, the immune system is perfectly capable with a normal diet, even a bit of a bad diet
The only way it gets better is from learning, hence a lot of people say let your kids play in mud so they can get the immune system to learn at a young age to protect you for life as you grow up.
As said before, create a new habit of not touching your face...washing you hands after being out at work..every time you go to the toilet, esp at work/ around kids, and clean things like keyboard mouse etc.