So far this year, 21 Coloradans have been infected with the Zika virus, although none contracted the virus while in the state, a county health official announced Monday.

Four of those Zika cases have occurred in Weld County, according to a news release from that county’s health department. All of the cases involved people who traveled outside of Colorado to areas where Zika is active, according to the news release.

A spokesman for the state Health Department could not immediately be reached to verify the figure. State health officials previously confirmed two cases of the virus in Colorado.

Weld County also has had seven cases of West Nile virus.

“The public health message is to avoid getting bit by any mosquitoes,” Dr. Mark Wallace, the executive director of the Weld County Health Department, said in a statement. “In Colorado, our mosquitoes are active at dawn and dusk; but in areas with Zika, the mosquitoes are active during the daytime.”

Although Zika and West Nile are both most commonly transmitted by mosquitoes, they are typically carried by different species of mosquitoes. The mosquitoes that carry Zika generally do not survive in Colorado’s climate, although there have been sporadic reports in the past of them here. A state Health Department infectious disease specialist said this month that no mosquitoes of the Zika-carrying species have been found in Colorado this year.

While Zika, which can cause birth defects, has received most of the attention this year, West Nile remains the more potent threat in Colorado. At least one person in the state has died so far this year from the virus. Most people infected with either virus, though, typically recover on their own.