How do doctors rule out the bad flu; my one year old nephew (premature baby); has been vomiting for two days and not wanting to drink his bottle ... has a very bad cough and they diagnosed him with croup; my sister has taken him to urgent care and Even the hospital Bc he isn’t playing and the vomiting doesn’t gotten better ; she has asked over and over, how do they know it isn’t the bad flu, the strain that could kill a baby or elderly and they just keep saying it’s not that. Well today she woke up vomiting and now my two year old nephew as well... question is, how does one know it isn’t just a regular flu .. she’s so stressed, afraid that it’ll spiral out of control and it’s not just a regular flu