Agree with many of the comments here, especially @electrontornado
Would suggest getting some blood work with your primary care physician and get your Vit D levels checked in the process. Vit D deficiency is extremely common, especially during the winter months, and some evidence of it playing an important role in your immune systems health. Supplementing at 1000IU a day is very safe, but it may take a slightly higher dose to get your levels back to normal if starting out low.
I hear you. Unfortunately (or fortunately) I’ve been supplementing Vit D for years at 1000 IU on my doctor’s recommendation due to my latitude at 42 North, and proximity to lake effect clouds . I might start upping it to 2000 in the winter. It’s the only supplement I take.
Also rarely get sick.
The times in the past ten years where I went through phases of getting sick more than usual have all been during periods of significantly increased life stress I was not recovering from. Either related to training volume on bike, or working 80+ hour weeks for months on end. etc.
When was the last time you took a two week vacation away from work or off the bike completely?
Also, COVID seems to cause some degree of immune dysfunction for a period of time after infection, potentially predisposing to increased URTIs.
I took a 3 week vacation last August, which was the longest trip I’ve ever taken since I’ve been in the corporate world.
My A event was last October 8th, and after that I took 3 weeks off the bike. I started my base program on 11/1, but (as noted) I was knocked out of the box over Thanksgiving weekend with the flu … and that is where the life began.
I had a rest week last week, but felt a cold settling in on Sunday … I took Mon/Tue off this week, felt well enough to try some endurance spinning tomorrow … but after 45 mins pulled the plug. No bueno.
From 2018-2019 I spent a collective 3 months outside the US and rarely got sick. I got Covid in late September '22 after not having any illness the previous three years of pandemic. Three weeks after symptoms anytime I got on to ride my HR skyrocketed at tempo paces with a large amount of fatigue. I ended up skipping Iceman in November because my fitness was gone and we just bought a new home, but now I am finally getting back to my numbers and have a new race scheduled and I feel another bug coming on. If it’s a complete derailing of training again - well, I might just go back to powerlifting…
Don’t do it! 
Count yourself lucky, rest up, and take a moment to look at the long game that is your annual training plan. Getting sick often sucks, trust me, I know. I’m the opposite of you in that I’m going to get sick 4-6 times a year, nothing major, just colds and the occasional stomach virus. I only had Covid once that I know of. I’ve dealt with frequent illness my entire life and friends, coworkers and partners have all remarked, “you seem to get sick more often than most” - yep.
It does seem that folks that rarely get sick are getting sick more frequently this season. Just know that you were quite lucky to go that length of time without very much illness. I suspect there will be an adjustment during this season and next as far as the frequency of illness in the general population.
Would have thought that’s obvious. Every time you mix with others your immune system gets assaulted by virus 
. There needs to be enough virus, an initial threshold dose of viral load above which it’ll cause sickness. Below that threshold viral load there will be an immune response , not enough to cause sickness, but enough to generate anti bodies and longer term memory of that virus.
Thus if you are constantly being assaulted by low levels of viruses, you won’t get sick, and when a bigger viral load of a previous virus hits you, your immune system remembers and is primed and responds quickly.
If you isolate and don’t get that low level exposure then when a sufficient viral load comes along you get sick. The low level exposure , just like vaccines increase the threshold of viral load at which you’ll fall sick.
Hold on to that idea of low level exposure when a kid looks you deep in the eyes and sneeezes straight in your face, after having just spent an hour licking all handholds in sight, and occasionaly, the floor 
I mean I never had that great of an Immune system, but since October it’s just shit and especially since December.) Had a cough that didn’t go away until the second week in January, and this week I have another one that keeps me from the bike. It’s frustrating especially since the next week is vacation and I will not be on the bike for that.
Yep - it’s immunological memory: Immunological memory - Immunobiology - NCBI Bookshelf
I’ve had a bit of a dig into this and found a few articles that essentially conclude the most likely reason is probably not immunodeficiency as such, but rather that long period of isolating, mask wearing, and very diligent hand washing hugely reduced people’s exposure. We also know that T-cells (essentially the first line responders) likely lose most of their ‘memory’ after ~9 months.
So the (very simplified) hypothesis now is that more bugs are getting past the first line of defence because the body is no longer immediately recognising them as hostile. They then get in, multiply after taking longer to be ‘spotted’, and so a more substantial immune response is needed (which makes you feel ill).
There was a nice analogy I read somewhere but can’t now find. Imagine a local bar has a doorman who’s worked there for years. He knows the local guys who look fine but are actually big trouble and simply turns them away at the door. When that bouncer goes on holiday and a new doorman without his experience takes over, he doesn’t recognise the dodgy blokes and lets them in. An hour later, he and 5 colleagues are breaking up a 10 man brawl and there’s glass and tables everywhere 
I had covid once and have declined all Boosters and masking…trying to keep the exposure levels up to keep the immune system primed. It’s been about 18 months since I had covid and I live with two elementary school teachers (wife and daughter), a high school student who plays soccer and swims(rides crowded bus to meets), and have 4 grandkids (ages 9 months to 9 years old) that I see weekly. Now, I confess that I take a daily multivitamin and extra vitamin D.
1000IU Vitamin D might be enough or have no effect whatsoever. Vitamin D supplementation, as far as I know, has to be done like this: test your baseline, supplement a reasonable amount daily, and test again after three months. Everybody starts from a different baseline and everybody has a different response to supplementation.
To understand what a “reasonable amount would be” you can follow this rule of thumb:
My N=1 experience. My Vitamin D was 22 ng/ml in February 2022 with no supplementation. Because my previous experience told me that I’m not a super-responder to Vitamin D supplementation I started supplementing with 6000 IU. I retested in July 2022 and I was perfectly in the optimal range at 48 ng/ml.
Of course, there is seasonality and sun exposure in the mix…
I think we’ve learned through the pandemic that illness is not a binary thing. There are degrees of infection and degrees of symptoms.
Imo you (and I) were never aware when we were harbouring disease either through low doses or unnoticed symptoms.
Fortunately both my wife and I don’t seem to be prone to any more infections than usual. At the first sign of anything or if we’ve been in contact with friends or family who subsequently are ill we ram this up our nostrils, squirt and inhale deeply. Who knows whether it works but Team Sky used to/still do use it to try and keep their team free of bugs,
Update: Feeling much better today … hopefully this time it was only a mild, 3-day setback. I had a recovery week last week, so m sure my fitness took a bit of a hit.
Getting back in the trainer today for a shakeout spin 
Good luck to all of you — stay healthy!
i think i kinda figured it out…
my exposure was somewhat consistent
2 days per week in the office (1 busy, 1 not so busy)
4 days per week at the gym (graveyard shift, less than 10 people in the premise, barely talked to anyone)
and then i increased it and added 2 x swimming at a university campus full of young people with their germs and own “microbiome”, especially after being hit with covid (for the first time too) in december probably caught my body by surprise.
I have 2 kids in school so i have a steady stream of things coming in for sure, let’s just hope my body can build back the immunity i had before, losing a month of training more or less kinda sucks
