What’s with people not understanding that they shouldn’t go around other people, especially small children, when sick!?

My sister was supposed to watch my 1.5 year old yesterday (Monday) because my mom was out of town and she normally watches him, and she was literally just going to show up and still watch him after having the flu all weekend! She didn’t even tell me she was sick, my mom is the one who reached out and said something to me about it. So then I texted my sister (on Sunday) and she said she had been sick all weekend but was starting to feel better except her stomach, and I’m like okay don’t worry about coming then, I’ll stay home from work. Then today (Tuesday) my mom said that my sister and her kids are over at the house hanging out with them.. does she not even stop to think that I don’t want my son exposed to their sicknesses still?? Just because they’re not actively throwing up or anything, doesn’t mean they can’t still be carrying something that can be passed on. I’m so annoyed at her 🙄🙄🙄 my poor son has already been teething really bad for a few days and had bad diarrhea, I really don’t need him throwing up on top of that and neither does he.