A new strain of the norovirus gastrointestinal infection called GII.4 Sydney is spreading quickly and caused 58% of norovirus outbreaks in December, up from 19% of norovirus outbreaks recorded for September, according to the Centers for Disease Control and Prevention.

Of the 266 norovirus outbreaks reported during the past four months of 2012, 141 were caused by the GII.4 Sydney strain, according at data collected by CaliciNet, a network of participating public health and food regulatory labs.

The spread of norovirus has contributed to a clogging of some emergency rooms also filled with patients with influenza-like illnesses in recent weeks, though CDC researchers say it is too early to tell if the new strain will cause an increase in overall cases of norovirus.

Norovirus is very contagious and is the No. 1 cause of acute gastroenteritis, which leads to diarrhea and vomiting, according to the CDC. More than 21 million people get norovirus each year, and about 800 die from it, according to the CDC.