WASHINGTON — Secretary of State Marco Rubio called the unexplained death of a Russian plague institute worker “cause for focus” but not alarm — as panic mounts amid speculation that the 28-year-old may have contracted pneumonic plague.

“We know what we know, obviously, about whatever the Russians have provided, which is, it seems that there was a worker at the lab that was infected, and this concern that could have spread, at least internally. So you’ve seen them do quarantines,” Rubio told reporters as he departed Washington for a trip to Europe.

“There’s very limited travel between the US and Russia, obviously.  But like anything else, something like that gets out it could certainly spread,” the diplomat added.

“So we’re watching and monitoring it closely. I don’t think it’s cause for alarm but it is cause for focus and a cause just to keep an eye on it.”

The woman worked at the Irkutsk Anti-Plague Research Institute of Siberia and reportedly died from pneumonia of unknown origin — prompting a quarantine of the hospital.

Pneumonic plague can spread from person to person via the air and is almost always fatal if not treated quickly with antibiotics.