I tend to think it's one of many stresses that can pile up your personal level, that your immune system can be shot.

And it's all too easy to NOT back off some optional stress when other forced stress is occurring, meaning you are open more often to a body not ready to fight and win against the germs.

We know the extreme examples can be bad, but I fully believe the smaller ones can eventually lead to same thing.

Like big deficit, terrible food choices, lack of sleep, overtraining - really shouldn't be surprised to have a body unable to fight off germs successfully.
Now because of those things, say you get allergy effect more now, or now in general, and that's a hard stress on system.

I think many of us with normally a reasonable deficit, pretty decent food choices, mostly better sleep, not training too much or it doesn't seem like it anyway - could still hit situations in life that add more stress.

Now the combined level of stress on the body is bad enough that really, if we knew, we should back off one of the other stresses.
But how often do we have stresses in life - and sleep better.
Or stressful situations going on - and eat better.
Perhaps we still try to squeeze in a hard workout, to make up for some missed ones.
Or we take a steeper deficit to make up for not getting to workout, or don't even pay attention to food levels and don't feel like eating during the stress.

If I don't back off quick, I've seen how long I can drag sickness on.

I'm one of those that does push the body to the edge teetering. Usually works. Sadly learned working night shift years ago, you can always make up for lack of sleep with food. One of those trading stresses. But only for so long.

Honestly I don't know the answer to that but I am starting to formulate my own opinions. In the last 40 years I have very, very rarely been sick. I worked twenty years, 10-12 hours a day in a factory setting and had taken only 5 personal days. And of of those 5 days, really only 3 were due to sickness, 2 were just 'sick days' I used for other reasons. I would get sick but for never more than about two days, and after it had run its course I was literally good to go. One great night sleep and I was damn near 100%.

I have been running a big deficit for about 6 months now. In that 6 months I have had a couple of minor colds, and a flu that I have been battling for about 10 days now. Never in my life have I ever had something like this linger, and linger, and linger. I just cant seem to shake this sickness. It is annoying and frustrating. It has affected my energy level, my motivation, my overall mood and outlook. I have put more chemicals into my body dealing with cough and congestion in the last week than I have the previous 40 years COMBINED.

Coincidence ? I don't know but it sure seems like my body dealt with sickness a lot better than it is handling in now. I know that in the long run being at a healthy weight and just being healthy in general in the best path but if this is the trade off I am not sure I can handle it. I honestly don't know how people can handle being sick more than once a year or for multiple days at a time.

Don't know if this got touched on, but cortisol, the stress hormone goes through the roof when calorie deficit has been a regular thing. Not only can this lead to easy fat storage but it seems to also be linked to depressed immune systems.

I am sure there have to be some knowledgable dudes out there who know the specifics but this is something I must have read somewhere at somepoint on some blog or another

Well that makes sense. Thanks for the information. Never would have thought of elevated cortisol levels.