My Son Returned From Mexico And A Few Days Later

my son returned from mexico and a few days later could not stop burping and then had stomach cramps. about 4days later, he became nauseous. i asked if he had any ice in mexico and he had. i thought giardia. he threw up after being nauseated and said something black and string came out. i dont know what that may be caused by. he felt better after throwing up but now diarrhea has started.