The story I’ve been told about the action of viruses is confusing me. They get into our bodies, take over the nucleus of a cell, and use our genetic apparatus to manufacture more viruses, which go out and do the same. What we feel during the course of a cold or flu is largely the result of our immune system, and that it’s possible that without an immune system, the first symptoms of a viral infection might be the faltering of some major body  system–heart failure or delirium, for example, if the heart or brain became compromised. Why do we have such a narrow range of symptoms? Fever, headache, congestion, sore throat, vomiting, tiredness, cough, maybe losing our voice–that’s about it, right? But why is there any variation, if what the virus does is the same? Why do we get a cough with one flu and not another?