Muscle aches caused by a flu virus cannot be accounted for by fluid loss from excretions and emesis alone, because often muscle aches are the first sign of the flu, long before vomiting or diarrhea may ever occur (and they do not occur in most flu infected folk). Instead, it is the increasing body temperature (the fever that is one of the body's most effective responses for killing off heat-susceptible invaders) that causes an increase in water usage at the cellular level. Full body dehydration then almost inevitably occurs, as the flu-infected usually have their thirst and appetite mechanisms decreased as well. However, as the previous writer puts it: One of the most notable symptoms of having the "flu"the is a persistant (and often disgusting!) loss of fluid. This can be a result of vomiting, diarrhea or often a combination of the two. Within this fluid are precious ions (electrolytes) that enable all the good stuff in your body to happen: Muscle contractions, nerve impulses, even basic cellular metabolism! One of the most important (as far as your muscles are concerned) is potassium -- [although calcium, magnesium, and sodium are all equally important but just dont get the same amt of press]. Low potassium levels mean special ion channels in your muscle cells cannot function properly, and that leads to a sensation of "exhaustion", much like you just finished a marathon. Without potassium (and other molecules such as ATP) your muscles just can't function! If you have the flu and you'd like to beat the muscle ache, chow on some high potassium-sodium ratio foods such as avocados or dark green veggies (think spinach); [bananas are merely ok regarding this K/NA ratio, much more important than is absolute amount of potassium alone]. Gatorade or other (preferrably lower in sugar) sports drinks can help both replenish lost electrolytes (like potassium!) AND water, a great two-fer when you're socked in with this bug (watch the sugar level or you'll end up low on potassium again, if you get my drift!).