In general, I only close if I am so sick I am unable to stand or if I'm throwing up. I would close if a close family member passed (or we thought was going to) as well. I've only officially closed maybe once a year. Almost every year it was for the stomach flu that was brought in by a daycare child. This year it was because all 7 children had a nasty cold flu. (1 got bronchitis, 1 an ear infection, and 1 pneumonia so it was a NASTY one!) and I closed to give everyone a day to rest and recuperate free of charge while I disinfected the whole house top to bottom. Officially none of them were well enough according to my illness policy to come anyway, so it was really a favor. They came back to printed out illness policy sheets waiting for them and a much "meaner" provider who started turning the sickies away again.
If I or my children (not my husband) are sick (by my illness policy), but I'm well enough to get the job done, I'll call all of the parents and give them the option of coming with the understanding that their children may be exposed if they bring them. I wash hands like crazy, take decongestants and cough suppressants and do my best to keep my kids separate on those days. Most parents opt to bring those days. You've got to figure, we are contagious before we are symptomatic, so they may come down with it anyway. If you close when your kids are sick and then they have to stay home when their kid gets it a couple days later, they've missed twice the work. Like I said though I give the option, so if they are lucky enough to have a friend, relative or neighbor that can watch them I don't charge for that day.