Female Aedes aegypti mosquito as she was in the process of obtaining a “blood meal” (credit: US Department of Health and Human Services)

The US Food and Drug Administration requested Wednesday that Florida’s Miami-Dade County and Broward County temporarily stop accepting blood donations after four people in the area inexplicably came down with Zika infections.

The four cases were not immediately explained by travel to an area experiencing an outbreak of the mosquito-borne virus, nor from sex with an infected person—the two main ways US residents become infected. This has led some health officials to speculate, though not confirm, that local mosquitoes may be transmitting the virus to residents in those areas.

If local transmission is confirmed, the cases would represent the first homegrown outbreak of Zika in the continental US. It would also suggest that Zika infections, which are linked to birth defects in pregnant women but otherwise mild illnesses in adults, may be spreading undetected among Floridians. Thus residents could conceivably donate blood containing infectious viruses without knowing it.