MINNEAPOLIS (AP) — The call is going out for blood donors in Minnesota and Wisconsin.

Memorial Blood Centers said its supply of O-negative blood is dangerously low across its 11 donor sites in Minnesota and Wisconsin.

Spokesman Larry Silber says the organization has been able to keep up with the demand for O-negative blood so far, but more donors are urgently needed. A combination of cold weather, vacations and the flu season has led to the decrease in donations.

The shortage is particularly critical because O-negative is a universal blood type that can be used in emergencies.

But O-negative donors are also rare. According to the American Red Cross, only 7 percent of people in the U.S. have O-negative blood.