Stanford University has issued an emergency call for all blood types—but especially O positive and O negative—because of higher-than-normal hospital usage and a seasonal drop in donations.

The Stanford Blood Center has been supporting a number of patients who need large quantities of blood, according to spokeswoman Loren Magaña. In the past week, five patients have required 500 blood products, including 200 pints of red cells.

“In summertime, many of our regular donors are traveling and we don’t have our high school and college blood drives to help bring in new donors,” she said.

The critical shortage reflects a nationwide trend. From May to June, the American Red Cross collected 56,000 fewer donations than usual, according to USA Today. It was such a dramatic drop that the nonprofit through the end of August will give an Amazon gift card to anyone who gives blood.

Magaña said donations to the Stanford Blood Center typically take only an hour and could save someone’s life.

“Please consider making an appointment to donate at one of our centers or mobile blood drives,” she said. “Now is the time, we need you.”

Donors should be in good health with no cold symptoms. They should eat a hearty meal and drink fluids prior to donating, and present a photo ID when they show up. To schedule an appointment, visit stanfordbloodcenter.org or call 888.723.7831.