Training whilst sick.. What do you do?
There's a general rule that if your symptoms are all above the neck then it's OK to partake in exercise. Should you have any symptoms below the neck such as a productive cough or achy limbs you should be resting. Exercising with a fever is a big no no.
The fact that exercise temporarily lowers your immune system has been partially debunked, however if you've been thrashing your body hard recently there's no doubt it'll be in a depleted state and it's ability to fight off a virus may be lessened for a time.
As with all things exercise related it's incredibly individual and there are many external factors that will have an impact on your decision to exercise.
For me this week I've been suffering with a head cold. For the first time ever I've been strict with myself and have halted the cardio in a bid to let the body focus on getting better. Mentally this is a challenge as I'm less than 4 weeks away from my big run, but I also have the pressure of support running for James Gibson from Saturday.
This is very much a case of short term pain (not logging the daily exercise in the sunshine) for the long term gain (being fit and able when it's vital to be). There are still other ways you can be productive towards your goal, such as sorting kit, route planning, researching or visualising.