The United States is currently facing a serious blood donation problem, as officials with the American Red Cross and others are warning.

This week, the Red Cross declared a severe blood shortage, following a sharp drop in the national blood supply over the past month. The winter cold and an especially hard-hitting flu season are heavily contributing to the shrinking supply. Eligible donors are being encouraged to visit their local center or blood drive as soon as possible.

“Every person’s blood donation can help save lives—plus you help ease the tremendous burden on our doctors and nurses, allowing them to focus on caring for patients,” said Courtney Lawrence, executive medical director for the Red Cross, in a statement from the organization.

A perfect storm

The national blood supply is often in a precarious state. Though roughly two-thirds of Americans meet the criteria to donate, only about 3% of eligible people do so annually. And since most blood products have a limited shelf life (just five days for platelets), hospitals constantly need new donations to replenish the supply.

That said, the winter in general tends to slow down donations. According to the Red Cross, about 400 blood drives were affected by extreme cold weather last December, more than threefold the number affected during the same time the previous year. Another crucial factor for the current shortage has been one of the worst flu seasons documented in recent history.

“This time of year is always challenging for blood collection—but right now, the perfect storm of a long holiday season, record flu activity and winter weather are all impeding donation efforts,” said Lawrence.

The American Red Cross and its centers provide about 40% of the country’s blood and blood products. But other organizations are feeling the heat, too. According to the latest tracking data from America’s Blood Centers (which provides most of the remaining 60%), 17% of community blood centers currently have a blood supply of zero to one day, while 34% have a blood supply of one to two days (3 days or more is considered enough to meet normal demands).

All in all, according to the Red Cross, the national blood supply has fallen by 35% over the past month.

Time to donate

Without enough blood, hospitals and doctors are forced to triage the limited supply available, which can lead to people not getting the optimal medical care they need, including in life-or-death situations.

So the American Red Cross is urging people to sign up and donate if they can. To encourage donation, the Red Cross is also providing giveaways and prizes, including a potential trip to this year’s Super Bowl. You can check your eligibility and look for donation centers in your area through their website or via their app.