It looks like flu. It feels like flu. It even puts patients into the hospital like flu can. There’s another virus out there that could be adding to the seasonal misery, but it’s not being identified. The virus is called adenovirus, and it can cause very severe flu-like symptoms. It’s so risky that the U.S. military vaccinates recruits against two major strains. But most people are not vaccinated against adenovirus, and doctor’s offices don’t test for it.