I'm not being a wimp, it turns out. Those feelings are a real thing called "sickness behavior," which is sparked by the body's response to infection. The same chemicals that tell the immune system to rush in and fend off invading viruses also tell us to slow down; skip the eating, drinking and sex; shun social interactions; and rest.
"Those messages are so powerful they can't be ignored," says Philip Chen, a rhinologist at the University of Texas Health Science Center at San Antonio. But that doesn't mean we don't try. Symptoms like a stuffy nose are obvious, Chen notes, but we're less aware that changes in mood and behavior are also part of our bodies' natural response to infection.