The seasonal flu typically keeps people from donating.
"It's usually around cold and flu season where I've got an appointment to donate, but I've got the flu, fever, that keeps me from donating then," blood donor Matthew Leong told ABC7.
Now, there is an added threat, the H1N1 virus. Blood centers harvest about half of the blood supply in the U.S. Nationwide, 27 percent of them are reporting a decrease in their collection because of the Swine Flu.
"We have seen a little bit of a drop off," said Lisa Block with Blood Centers of the Pacific. "Our blood drives aren't quite as busy. People aren't calling quite as much to make appointments. So, we are concerned that it is going to affect our blood supply."
Even on a good day, the Bay Area imports 20 percent of the blood it ends up using. In the upcoming months, the demand could be greater. Some donors call a day or two after giving blood to report a fever or flu-like symptoms. When that happens, the blood they donated is thrown out.
In the small chance that the virus was not detected, the blood used during a transfusion should still be safe. There are viruses that could spread from a transfusion including HIV AIDS, Hepatitis C and West Nile virus. Colds and the flu have been the exception.
Dr. Philip Norris does not expect the Swine Flu to act any differently. He has done research on blood safety funded by the National Institutes of Health.
"We've tested about 50 of those patients so far and found no evidence of influenza virus in any of those patients," he said.
Block says, "That why it's so important for people who are healthy to donate blood, to do it now."