Long-term care facilities are seeing the worst of a nationwide outbreak of a potentially dangerous new strain of norovirus, the Centers for Disease Control and Prevention announced Thursday.

The GII.4 norovirus strain, called GII.4 Sydney, was first identified in Australia last spring. It has spread globally, and accounted for the majority of norovirus outbreaks in the United States between September and December, according to the CDC. Of the 141 GII.4 Sydney outbreaks during those months, 65% occurred in long-term care settings.

The proportion of norovirus outbreaks associated with GII.4 Sydney surged from 19% in September to 58% in December, the CDC noted. This is bad news for long-term care providers already grappling with a bad flu season, as experts say GII.4 noroviruses generally cause more hospitalizations and deaths than other norovirus types. Like other noroviruses, symptoms of GII.4 Sydney include gastrointestinal distress.

Healthcare professionals warn that GII.4 Sydney is highly contagious.