Most parasites that like to live inside cells are sly opportunists that trick cells into letting them in. The unsuspecting cell swallows the parasite as if it were food. Researchers have suspected that Toxoplasma, a protozoan that can fatally infect people with weakened immune systems, such as newborns and AIDS patients, has a different modus operandi. Cells infected with Toxoplasma don’t show a ruffling membrane or any other signs of having swallowed a parasite. It looked as if it was slipping in without any response from the host, says microbiologist David Sibley of Washington University. He and Janice Dobrowolski have shown that Toxoplasma doesn’t dupe a cell--it forces its way in, as shown here. The researchers found that Toxoplasma attaches to a cell, forming a ring-shaped junction. The junction moves from the front to the rear of the protozoan, probably powered by actin and myosin protein filaments, like those in human muscle, constricting the protozoan and pulling the host membrane over it. It’s somewhat like pulling a sock over your foot, Sibley says. He thinks that related parasites, like those that cause malaria and dysentery, use a similar method. Now we know the motor they use to invade, he says, so we have a potential target to think about disrupting.