For years, health officials in the US noted sporadic, mysterious cases of a foreign bacterial infection, called melioidosis. The infection—which is difficult to diagnose, tricky to treat, and often deadly—was thought to only strike travelers or those who came in contact with contaminated imported goods or animals. Yet, now and then, an American would inexplicably fall ill—no recent travel, no clear links.

Now, health officials have a definitive explanation. And it confirms a dreaded, long-held suspicion: The deadly bacterium is foreign no more. Rather, it’s a permanent US resident entrenched in American soil.