Toxoplasma gondii is the most widespread parasite on earth, found across all continents and in a staggering variety of habitats. We have only recently discovered how many different animals it infects. To the surprise of many, University of British Columbia scientists found the parasite in beluga whales in the Arctic in 2014. Off the California coast, Toxoplasma infection has been revealed to be responsible for sea otter deaths formerly attributed to sharks or boats.

The wily intruder owes its success in part to a high rate of expansion. A cat newly infected with Toxoplasma can excrete up to 800 million packets containing the parasite—called oocysts—in the span of about seven to 14 days. Once released, an oocyst can persist in soil or water for years. Inhaling or ingesting just one oocyst is enough to establish infection, which virtually guarantees that the parasite will find its way into a variety of new hosts.

But house cats may have unfairly gotten a bad reputation from their link to Toxoplasma. Pet owners may indeed accidentally ingest an oocyst if they fail to wash up after tending to kitty litter, but scientists believe an individual cat will shed oocysts only once in a lifetime.

In contrast to housebound felines, wild and feral cats can spread oocysts in gardens, farms and water reservoirs, meaning they are likely to be greater contributors to Toxoplasma's spread. Oocysts are commonly found in dirty water or on unwashed fruits and vegetables. The parasite does not infect plants, but its oocysts can remain on their surface unless they are carefully cleaned.

Moreover, many animals—including humans—are likely to become infected by eating raw or undercooked meat. Any animal infected by Toxoplasma will harbor tissue cysts for the rest of its life. When that tissue is eaten in, for instance, steak tartare, the para-site spreads into its new host. These clever methods of entering new hosts have allowed Toxoplasma to disseminate across the globe.