A friend of mine fell ill with COVID-like symptoms; bad enough to be unable to work remotely, but not bad enough to go to the hospital and be tested (in the UK). He was sick for 5-6 days, then felt better, and thought he'd completely recovered. One week later, his symptoms returned for 2 days, and now he's feeling better again.

Was it COVID the first time? Was it COVID the second time? Was it stress, or his body adjusting to being in the same environment 24/7 (rather than spending half his day outside the home)? We may never know. An antibody test would show if he'd had it at least once, but that could well have been 2 back-to-back colds with asymptomatic SARS-CoV-2.

If it is only a few days later, it might be the normal covid19 pattern. I have been warned by a doctor of this two steps pattern when I got it, which I understand corresponds to infection by the virus first, then bacterial infection as a result of the damage caused by the virus.

Not a doctor, but just my understanding from [1] though I start seeing people challenge that pattern [2].

[1] https://www.youtube.com/watch?v=BtN-goy9VOY

[2] see responses to: https://news.ycombinator.com/item?id=22786656

Is it unusual to seemingly recover from a disease, and then relapse into more severe symptoms? I've seen that happen hundreds of times with less serious illnesses (colds, etc). Especially if the sick person stops taking it seriously and starts doing things like drinking alcohol, etc.

Stands to reason that it would apply to COVID-19 too.

Sometimes it doesnt matter what the person does.

HIV - https://upload.wikimedia.org/wikipedia/commons/thumb/0/0e/Hi...

It takes time as someone above said to work out whats happening and what to do - https://www.youtube.com/watch?v=Z48SZykpQeY

People forget a flu (especially in the past) could take weeks to recover from. Easily 2-3 weeks.

I Wonder if a virus could create waves inside one's body

From what I've picked up separate waves describe what is happening in typical Covid-19 patients: immune reactions that are good enough against the virus in the throat are insufficient to defeat it elsewhere so you get a first wave in the throat and if the virus spread into the lungs before that wave is over there's a second wave in the lungs. That wave then requires a different or stronger immune reaction.

For the flu, absolutely. It mutates rapidly and one can get the virus, mount an effective defense, and then the virus mutates and escapes the immune defense. This happens. It is known that coronaviruses mutate much more slowly than influenza for fundamental reasons. It is highly probably but not proven that this much slower rate is sufficient to preclude the "waves" you refer to.

It is a totally unrelated virus, but if you get chickenpox you may get many years later shingles, the virus may remain inactive in nerve cells fo a long time. https://en.wikipedia.org/wiki/Shingles

What a strange game of life

Or the reverse, tests returning to negative before symptoms kick in: the tests only check for presence in the accessible part of the throat and that area is known to get negative after a few days (apparently the virus is easy to defeat there). An infection down in the lungs could still be in early stages and growing at that time so symptoms would later appear as if out of a (recently clearedh clear sky.

Or were one or both tests a false positive?