If you get the flu, you likely experience fatigue and malaise, aches and chills, coughing and sneezing. If you are wise, you stay in bed and withdraw from social activities. These symptoms of infection are (unfortunately) familiar to all of us, because they are stereotyped —that is, they happen to all of us with more or less the same pattern when encountered with a pathogen such as influenza. Such symptoms of infection are also conserved throughout the animal kingdom, which gives us insight into their natural selection. What you may not know, even if you are well versed in science or medicine, is that these "symptoms" are not side effects of infection at all. That is, they are not initiated by the influenza virus itself, which would much prefer to remain undetected within its cozy host. Rather they are evidence of detection and retaliation by your immune system — these "sickness behaviors" are organized, adaptive strategies orchestrated by your own immune system and are critical for host survival. To put it bluntly, your own immune system is making you feel terrible, in order to make you better.