The Gambian pouched rat is not a rat scientifically speaking, but the term "rat" is close enough for everyday use. It is as closely related to mice and hamsters as it is to rats. Speaking of hamsters, it gets the "pouched" in its name from its pair of hamster-like cheek pouches.
The Gambian pouched rat is a ROUS (Rodent of Unusual Size), being about three feet long including its tail (which is half its length). Like many rodents, it is inquisitive, social, and surprisingly easy to domesticate. It is commonly kept as a pet in Africa, where it is native to the continent.
This large friendly rodent became popular in the United States in the late 1990's as an exotic pet. Can you guess what happened next? It escaped, and it made people sick.

Escape of the Gambian Pouched Rat
No one knows where the first one came from. Was it released by an owner who didn't want it? Did it escape? Was it brought to Grassy Key, or did it swim there from somewhere else? Or did it perhaps hitch a ride on a ship?
At any rate, in 2006 a researcher confirmed that a breeding population of Gambian pouched rats was present on this island in the Florida Keys. Because the Gambian pouched rat is large, quick to breed, competitive, and omnivorous, it poses a significant threat to the Florida ecosystem. If the rat makes it up to the mainland, it could spell doom for the Everglades.

A Tiny, Rat-Sized "Achoo"
In 2003, the CDC noted an outbreak of monkeypox, an infectious disease which (despite its name) more commonly strikes rodents than monkeys. It is common in "remote villages of Central and West Africa in proximity to tropical rainforests." It is similar to smallpox, but not as lethal.
Almost a hundred people came down with monkeypox in 2003. Most of the cases were traced back to pocket gophers, but the CDC banned importation of the Gambian pouched rat just to stay on the safe side.

HeroRATS
Unexploded land mines continue to be a significant problem in Africa. As does tuberculosis, which kills about 1.8 million people a year worldwide. In Tanzania, an organization called APOPO is training Gambian pouched rats to detect and indicate both of these problems.
Thanks to their phenomenal sense of smell, and their trainability, Gambian pouched rats can easily detect both buried land mines and tuberculosis in sputum samples. If APOPO is successful, the humble Gambian pouched rat could become a literal savior of humanity!