Summers usually see blood banks running short on supplies — there simply aren’t as many blood drives, and the drives that are held see fewer donors showing up.

This year however, things could get much worse, due to concerns over the Zika virus.

Most blood donation centers would be putting out the call right now to all available donors to come in and give blood, but the American Red Cross this week put the following notice on its website:

ATTENTION: The American Red Cross is closely monitoring the Zika virus. We ask you not to donate if any Zika virus risk exposures apply to you. We are asking you to return to donate blood 4 weeks after your last risk exposure. All other eligibility requirements still apply.

They’re not alone in being cautious. Blood banks everywhere are on high alert.

Anyone who may have visited a location for which the Centers for Disease Control and Prevention has issued a Zika travel notice — the Caribbean, Central America, South America, the Pacific islands, and Mexico — must wait 28 days before donating. 

The concerns are also growing about the Aedes mosquito making its way further into the heart of the U.S.

The White House continues to seek $1.9 billion in new funding from Congress to fight the virus. House Republicans are instead proposing measures that would support the immediate initiatives of vaccine development and mosquito control by reallocating funds from other existing programs. A vote is expected in the House on Tuesday.