is it true....

that when you are sick, you need more calories to get better?

 'Invalid food' aimed at sick people confined to their bed with little appetite is traditionally light, easily digested and nourishing.... chicken soup, calves' foot jelly, stewed apples, glucose drinks.   Someone recovering from an illness can cope with more heavy-duty foods and, yes, probably more calories to replace their lost energy.

but what if it's just the flu or another short-term illness (i.e. lasting no more than a week)?

thanks for the info!

If you've had a virus that's laid you up for a few days and put you off your food then you'd naturally eat more once you felt better to get your strength & your appetite back.  That would be pretty normal.  If you're asking 'how many more calories should I have because I've been sick?' and looking for something more precise I don't think calories really come into it.   

i really think this depends on the person. I have no appetite when I'm sick, eat as I normally do when I'm not anymore and get better in a fairly expected amount of time. I'd just go with how you feel and not worry too much over what you "should" be doing.