All mammals get fevers, and even sick lizards instinctively head for a hot rock. People, on the other hand, head for the medicine cabinet. Sharon Evans and her colleagues at Roswell Park Cancer Institute have found out what it is we’re messing with when we casually bring down a fever.

To see how body heat influences ­infection-fighting white blood cells, Evans put mice in a hot box that brought the mice’s temperature up to 103 degrees, then injected fluorescently dyed white blood cells into them.

Compared with unheated mice, the animals with the faux fever had twice as many white blood cells migrating out of the blood vessels and into the lymph tissue that lines the skin and gut, which is where they need to be to attack incoming pathogens. “A fever can help rather than hurt you,” she says.

Physicians have long suspected that high temperature must somehow ramp up immune responses. “Hippocrates used to routinely treat patients by trying to heat them,” Evans says. Evans’s study finally closes in on some of the molecular effects of a fever. Heat increases the density of sticky molecules on the surface of blood vessels in the lymph nodes, which catch more white blood cells as the blood rushes past, Evans says. “It’s a cellular Velcro kind of thing.”