Really really sick, what could it be?

My boyfriends sick & im pretty concerned but idk what it could be.. He recently had a cousin start living with him & he said since yesterday he got sick & he thinks its from her. She's also been sick but its not flu or runny nose typa sick. Its a real bad stomach ache i guess, she feels like she has to throw up, she has thrown up, she's went #2, all that. My bf has the same and i was trying to figure out what it was, but idk what it could be? Does anyone know? I jus got off the fone with him & he said he used the bathroom & its kinda diarrhea, he tried to throw up bcuz he feels like he has to, and he tried eating but it dint help, hes taken pepto but it hasnt really worked. If n e one knows, that'll b great. Thanks alot.