If you're training for a fun run or have a solid exercise routine, there's nothing more frustrating than feeling you're coming down with a cold or flu.
Should you push on with your normal exercise load in the hope it might help your immune system shake off the illness?
Or will that stress your body and make it worse?
It's an issue even elite athletes face, says David Pyne, a sports scientist and researcher with the Research Institute for Sport and Exercise at the University of Canberra.
In general, exercise helps boost your immunity, but pushing yourself too hard can temporarily have the opposite effect.
Add a looming illness into the equation and it's trickier still, so athletes use a test known as the "neck check", Professor Pyne says.
In general, if symptoms are from the neck up and not too severe, moderate exercise won't hurt you and might even be beneficial.
"You might have a sore throat or maybe a runny nose or a slight headache," he says.
"If symptoms are fairly mild, it's probably OK to go out and undertake some [low- to moderate-intensity] exercise if you're feeling up to it."
When to skip the gym for rest and recovery if you're unwell
If you feel unwell and your symptoms are more troublesome, particularly below the neck, pushing on is not recommended.
"We're talking about chest congestion, or any muscular or joint aches and pains, or a temperature," Professor Pyne says.
"They're what we call systemic symptoms; they affect the whole body. And then the smart advice is not to exercise."
Exercising with major cold symptoms, particularly a fever, can prolong your illness and be dangerous.
It can make you faint and, in rare cases, even cause heart damage.
Also, if you're training in a structured way to meet certain goals, say in the lead-up to a fun run, the quality of your training counts as much as the quantity; training when you're sick will not be quality training.
While it's easy to fret about all your hard work being undone by a break from your exercise routine, missing a few days or a week is not going to have much impact if you've been exercising regularly before that, Professor Pyne says.
"You don't tend to lose too much [fitness] unless you've had a few weeks off," he says.
Come back to exercise gradually after being sick
But what about getting back into things after a bout of illness?
Most common colds are self-limiting, with symptoms resolving after a few days or a week.
Once you've got through that period and any aches, pain, cough or fever have passed, it's fine to start exercising again — but Professor Pyne says it's important to do it gradually.
"We often advise athletes to ease their way back in with light exercise on the first day, then gradually step up to full training," he says.
"If they've only had something mild, it normally only takes them a day or two to get back [to their normal routine].
"But if they've missed a week, then we'd normally allow two to three days to get going again."
Professor Pyne suggests the same applies to recreational athletes but says it's important to monitor how you feel and don't try to push yourself too hard to make up for lost time.
If you feel you're someone who tends to succumb to infections easily, you might want to be a bit more cautious.
Research by the Australian Institute of Sport has shown about one in seven, or 15 per cent of, active individuals are more prone to infections than others.
Likewise, about 10 per cent are fortunate souls who seem to never, or very rarely, get sick.
Exactly what makes these people damned or blessed isn't known, but it's likely that genes influencing individual physiology and lifestyle factors like diet, sleep and stress are involved.
You need to learn to "read your own body", Professor Pyne says — which might take some trial and error.
"The thing is not to rush back and do too much too quickly," he says.
Article from ABC Life