After an SHTF event occurs, several common things come into play. People lose sleep. They live in crowded unsanitary conditions. They eat foot that isn't perfectly prepared. Clean water is difficult to find. There is less availability of hot water and soap. The norovirus or Norwalk virus is a very common result of living in close quarters under these circumstances. It's the most common form of gastroenteritis
The symptoms are often severe watery diarrhea (not bloody) and vomiting. Everyone has had a bought of vomiting and diarhea, this is far worse than then norm. The person feels weakened and out of sorts. They shed the virus, and anyone around them will probably get it despite renewing efforts to clean up more thoroughly. The virus may stick around for several weeks too.
You can get reinfected. Having it once is no guarantee that you won't get it again. Ordinarily it lasts for three days, but under crowded conditions you might easily acquire it again.
When a disaster strikes, it's not like normal conditions in which there is a ready supply of electrolytes and medical attention. Food and clean water may be very scarce, and that declines as time goes on. Expect children and elderly survivors to get it, and then more and more healthier people to acquire it as well. If you've ever seen it or had it, you know just how crazy things become. If you have many people sharing a common bathroom, or living in rudimentary shelters, then you know it will happen.
There is no cure for a virus, it has to run it's course. Despite all the diarrhea cures, they really don't work. What they do is slow down a spastic intestinal system to allow people's systems to calm down. In this case, it's better to allow the virus to be eliminated within reason to get it out from the intestinal tract.
The only way to really help is to isolate the sick person as much as possible and have infected people use the same bathroom versus another one for healthy people. They also shouldn't be around food being prepared nor help in the kitchen at all.
This means you have to be vigilant about food prep and sanitation. It's very likely that during the course of it through a household, that the strongest person who's the primary caregiver will end up with it last, right when they're taking care of everyone.
[link to en.wikipedia.org
[link to www.medicinenet.com