(NBC News) A nasty new strain of norovirus, a highly contagious gut bug, has circled the globe and landed in the U.S., where it’s now the leading cause of what’s known indelicately as “winter vomiting disease.”

Health officials at the Centers for Disease Control and Prevention said Thursday that the GII.4 Sydney strain of norovirus was responsible for more than half of outbreaks of the illness during the last four months of 2012. The new norovirus spread amid a particularly harsh flu season that’s also causing misery.

Of 266 outbreaks of norovirus between September and December, 141 were caused by the bug that was first detected in Australia in March 2012, according to data from CaliciNet, which tracks norovirus outbreaks. The proportion of outbreaks caused by the new strain jumped dramatically from 19 percent in September to 58 percent in December, the CDC says in its weekly report on death and disease.

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