Antonio Barragan and his colleagues at the Karolinska Institute in Stockholm were puzzled at first about how Toxoplasma managed this swift journey. "When we looked for parasites in the blood, we found very few that were just swimming around," said Dr. Barragan, an associate professor. But the scientists observed many of the parasites inside immune cells known as dendritic cells.

Dr. Barragan was intrigued. Dendritic cells, common in the gut, often come into contact with pathogens. They respond by crawling to the lymph nodes or the spleen, where they communicate with other immune cells.

"That led us to think, what if this parasite is directing these cells to move and to disseminate through the body?" Dr. Barragan said. He and his colleagues put dendritic cells in a dish and injected them with Toxoplasma. They noticed that the parasites triggered a peculiar change: the dendritic cells became hyperactive, crawling for an entire day.

Next, the scientists observed how Toxoplasma spread through a living animal. They added a firefly gene to the parasites so that they produced a glow. When they injected the parasites into mice, a little of the light escaped from the animals. By putting the mice in a darkened box, Dr. Barragan and his colleagues could track the parasites as they spread.

Injecting dendritic cells carrying Toxoplasma spread the parasites to the brain and other organs far faster than injecting Toxoplasma alone. The researchers concluded that Toxoplasma was taking charge of the dendritic cells and riding along with them. Their results are published online in the journal Cellular Microbiology.

As Toxoplasma spreads through the body, it invades cells. Unlike other pathogens, Toxoplasma can enter almost every type of cell in the bodies of thousands of host species. The parasite slips into a cell by latching onto its surface and pulling the membrane over itself. "You can think of it like sticking your finger into a balloon," said Vernon B. Carruthers of the University of Michigan.

David Sibley, an associate professor of molecular microbiology at Washington University in St. Louis, said, "It just sits there, and the host doesn't recognize it as a foreign body it should destroy."