REST

A few months ago I was asked by a girl on a high school water polo team about training when you get sick. A few weeks after that her mom came to the beach looking for me and she wasn't happy. I like blogging this stuff because I can be more clear than this girl was to her mom. So listen up, you two.
One of the most important basics to understand for athletes, coaches and personal trainers is heart rate, both resting and maximum. Although it does vary depending on gender and age, the max heart rate isn't much different in fit and unfit people (but you can be sure that fit athletes have hearts that are much stronger and pump more blood.) The resting heart rate in fit athletes is much lower.
Rest occurs in the human body when the heart is beating at 50% or less of the maximum. Rest means that tissue is being repaired, glycogen is being replaced (short term energy storage) and muscle growth is taking place - basically all the growth and repair functions.
A fit athlete might have a resting heart rate of 50 and an overweight-out-of-shape sedentary type person might have a resting heart rate of 85. Both have a max of 190 and the goal for rest is to stay under 50% of that or 95. The athlete has 45 beats per minute to play with and the out of shape guy only has ten.
When you get sick your heart rate goes up. When you are feeling stressed it goes up. When you exercise it goes up. If you have a high fever it goes up a lot. So, short of a high fever (when you should pass on the exercise) yeah, get out there anyway but just cut back the heart rate to 50%.
Bad news though for the couch potato. Mild stress will spike his heart rate up to 95 and then even that walk to the fridge is over the limit. And when he gets sick it's really bad news because his body can't rest and never fully heals. You can imagine what happens next . . . unfortunately you don't have to - just look around. If you just can't seem to get enough rest, if you're always tired you're just going to have to get that resting heart rate down. EXERCISE.