Can you share a story about the party where many ended up sick?

I went to a child’s 3rd birthday party last Saturday. 17 people were there.
On Monday morning, at least 6 were incapacitated with the flu, including me. That’s a full 3rd.
My sister, who threw the party, is mortified. She knew her grandson was getting over the flu, but the doctor told her that he would not be contagious. Whoops.
Has anything like this happened to you?
What caused it?
Did that hamper later attempts to throw a party?