UKC

I had the flu about 4 weeks ago. Really bad. Started recovering about 3 weeks ago, and feel pretty normal this week.

I've only been running a year or two, so not gone through a illness recovery yet of this magnitude and it's got me curious.

I've been for a couple of runs now, and my god, my heart is up at 165bpm doing gentle 13 minute miles. And then I need to walk a bit. So whilst I thought I was recovered, I'm clearly not.

But what's going on here?

Light reading on the subject says my immune system may still be recovering and my body is low on energy, but I don't know what this means.

One interpretation is my immunise has sacrificed itself and it needs to re-build it's cells which appears to be a slow process. So when I run I'm competing for recourses.

Another interpretation is that my body has lost something it needs to run. Maybe it ingested all my mitochondria as a handy source of protein to fight the virus and I need to rebuild them all.

Just curious if anyone know a scientific explanation why exercise is so hard after being ill.

I wouldn't want to comment on the specifics, but I was listening to a programme on Radio 4 about long Covid. They showed that the tiredness was associated with muscle damage (with some very alarming numbers for a situation which is different from yours). Take home message was don't try and do too much. Pushing it can slow recovery. They did make the comment that fatigue after non-Covid virus infections may be related.

The flu virus lives mostly inside your cells. In order to clear the infection, your immune system must kill infected cells and clear the debris.

Your body has two types of immune response. Innate responses can happy very quickly, but are not very specific. They may cause damage to healthy cells alongside the infected ones. Innate immune responses often act to prevent an infection getting a toe-hold, or to keep it under control until the adaptive immune system can act.

The adaptive immune response is much more specific - it will explicitly target infected cells. However, it takes 7-10 days to become fully active if faced with a virus or other pathogen it hasn't seen before.

Thus, during a flu infection, some of your normal cellular functions are likely to be upset by your own immune system as it attempts to clear the infection, particularly before the adaptive immune response is fully active. The nature of the damage caused by that can vary, but it can include damage to muscle (including heart muscle), increased permeability of vascular systems, dysregulated clotting and a host of other things.

Any one of these things could be contributing to the difficulty exercising you describe. Often, specific causes in any given case are unclear. However, damage to muscle, including heart muscle is common with flu.

Don't know about the science but the main messages are

1. You're only fully better a long time after you feel fully better
2. Don't push it too soon even if you feel ok, that way lies post viral and CFS/ME complications - a road you want to avoid

You may be aware of that already, take your time, listen to your body, get well 

Bacteria make you sick by excreting toxins (either actively - exotoxin; or by dying and bursting open - endotoxin.). So really your body 'just' needs to clear up the toxin and the cells they've made sick.

As mentioned by Kathryn, viruses are not really alive as such (plenty of semantics here) and are a few proteins with a bit of genetic material in the middle. They can't replicate themselves (like cells - either bacterial or animal - can), but instead coopt the machinery of living cells to make lots more viral particles. They then burst out, killing the cells in the process before moving on to infect others. They can also produce toxin-like deleterious agents to if memory serves. So here your body needs to clean up dead cells, nasty bits and infected cells (which is difficult as the viruses are hiding in your cells) and then replace them. The clean-out and cell replacement, I assume, takes longer than just clearing out bacteria/toxins. 

When harnessed by science some of the stuff you can utilise bacteria and viruses (and animal cells) for is quite mind-blowing with medical applications - and that's coming from someone ho's always studied/worked in the area.