What Rick says here is standard fair when it comes to the legal protections but do understand that if an amicable agreement is not reached withing 30 days, you do need to refund the deposit (which should be a minimum of 35% of your fee/ 50% is normal)
HOWEVER, having the sniffles doesn't count. . . having cracked ribs or wearing a cast doesn't count. . . I've performed with my ribs wrapped and leg in cast, with a neck support collar and feeling like I was going to die on more than a few occasions. As the saying goes THE SHOW MUST GO ON . . . while agencies might excuse you once in a blue moon, they never forget. If you cancel on them too many times (more than once over a two year period, for example) and the reason isn't a matter of life or death, they will stop handling you and worse, will not likely offer many kind words on your behalf. So it is our bane in life to keep on trudging.
I've personally had to stop booking shows too far in advance for this very reason; I don't know from one day to the next and sometimes from one hour to the next if or not I'm going to be in the ER by night fall. It's quite frustrating even though I still ply my hand at doing shows here and there. Unfortunately I'm unable to deliver the sort of quality I use to pride myself on.