I'm not excluding other possible causes of crashes, but those two seem most likely in my case.
A normal person who doesn't have ME/CFS might think, "we all know what a cold or flu virus feels like. Can't you tell if you feel like you have a cold or a flu?" The answer is "no" because one's immune system changes when he/she contracts ME/CFS. It reacts differently to pathogens -- even ordinary cold and flu viruses. This change is often described as an imbalance between two sides of the immune system: Th1 and Th2. The famous ME/CFS doctor Cheney describes the problem (as transcribed by one of Dr. Cheney's patients, with permission):
Researchers have demonstrated that most CFIDS patients end up stuck in Th2 mode. This has several consequences. When the Th2 system activates, it blocks the Th1 system. This suppresses the Th1 weapons, particularly NK function. Accordingly, there is also an increase in the Th2 weapons - the white cells and antibodies. Most notable is increased antibody production. Dr. Cheney said that if you measure antibodies to anything a CFIDS patient has ever been exposed to, they will very likely be elevated....
Cheney notes that other problems ensue. Patients get into trouble on both sides: they overreact to things on the right side and under-react to those on the left. When they are Th2 activated, they no longer have the defense mechanisms to keep dormant all the things they caught in the past. They cannot suppress or control them anymore, and the EBV, chlamydia pneumonia, CMV, etc. reactivate. The yeast also begins to appear.
The only defense against being eaten alive at this point is RNase L....RNase-L cannot kill any of these things. It only stops them from reproducing. According to Cheney, "It's a line in the sand saying 'No more replication', and it waits for Th1 to come and kill them. But Th1 never comes. RNase L sits there and grinds away, possibly going up and down as the pathogens activate and reactivate. But they never get wiped out. RNase L holds the line, waiting for the cavalry that never arrives.
The Th1 side of the immune system is the side of the immune system responsible for, among other things, attacking and clearing pathogens that are "intra-cellular"--that live and replicate inside human cells. The Th1 immune cells know how to find and destroy viruses that hide inside cells. Even ordinary cold and flu viruses fall into the this category of intra-cellular that replicate inside cells, meaning a person with a Th2 dominant immune system would have a harder time clearing these viruses.
When a Th2 dominant PWME contracts a cold or flu virus, the virus still triggers the immune system. But it triggers the immune system in a new and different way. Other aspects of the immune system try to compensate for the deficient Th1 response. Th2 is turned on (further) when Th1 should be turned on. So when a Th2 dominant ME/CFS patient contracts a new virus, we don't feel the same as when a normal person encounters a new virus. Our immune systems becomes triggered, but in all the wrong ways. These "wrong ways" make us feel horrible - but not "horrible" in the same way a normal person feels when they get a cold or flu. It's much different. And it lasts longer. It comes with neuorologial symptoms like brain fog, muscle twitches, numbness, and a long list of other oddball symptoms.
Because of this, I often can't distinguish between the two types of crashes I described in my first paragraph. They essentially feel the same -- with the possible exception that a cold or flu- triggered crash might bring more typical symptoms like sore throat, and runny nose, in addition to the "oddball" symptoms. On the other hand, I can't rule out that a "rally" type crash might also bring on those symptoms.
Sometimes I wonder if all of the crashes I've ever had fall into only one of the two categories. I have this pet theory that I wonder about sometimes. Maybe all of the ME/CFS crashes that I've ever had are due to me encountering run-of-the-mill every-day viruses that my weak immune system struggles to neutralize. We all (healthy and sick people) people encounter viruses nearly every day, but a healthy person's immune systems manages to clear most of these viruses without the person every feeling symptomatic. It's only the occasional, more robust virus that makes the person feel symptomatic. It's possible that having ME/CFS--at least my version of it--is simply a state of having my immune system constantly triggered in a defective way.
(For many of my ME/CFS friends who react to mold or other environmental triggers, it's essentially the same thing except the catalyst that triggers their immune systems is different. Rather than being viral, it's environmental.)
The only way I could test this theory would be spend time in a hermetically sealed bubble like the famous Seinfeld "bubble boy." My suspicion is that if I ever spent time in an environment free of pathogen exposure, I'd feel great. Since that's obviously not realistic, I'll just have to wonder...