It all depends on what's wrong with you say knowledgeable medical folk - even Millie Mackintosh takes a day off when she's feeling rough as toast!
Listen to your body
"We all know that exercise is key to good health, but there are times that your body may need a break," explains Keith Veselik, Director of primary care at Loyola University Health System.
"Having to slow down when you're sick is Mother Nature's way of saying don't push it and it's reasonable to pay attention to that."
If you're body is battling an illness – it is already burning up energy in the fight, so your body might not be able to take the extra stress.
What kind of ill are we talking?
Speaking to Science Daily, Veselik explains there is a simple rule to follow – it's ok to exercise if the symptoms are above the neck, such as a runny nose or sore throat.
If you have fever, shortness of breath, chest congestion, body aches, diarrhoea, vomiting or feel dizzy – crawl back under the duvet.
Manage your expectations
"If you aren't feeling well, but still want to exercise, lower your expectations about what you can do," said Veselik.
"You don't necessarily need to be in bed all day, but you can't expect to have the same level of energy as you would if you weren't sick."
Is it OK to hit the gym?
Think about where you're working out. Avoid the gym or fitness classes in case you pass on your germs.
Instead go for a walk or a run by yourself. But remember to wrap up warm!
If you have any concerns about working out when you're ill, always see your doctor first.
EVERYTHING WE KNOW ABOUT GETTING A FLAT STOMACH
HOW TO SLEEP BETTER TONIGHT
MORE GYM AND HEALTH ADVICE