Mosquitoes known for carrying dangerous diseases such as yellow fever and dengue were found again this week in Fresno after groups had been detected several times throughout the summer in California, according to The Associated Press.

The first wave of Aedes aegypti mosquitoes were found in the central valley in June and the second spotting came in August in the bay area, the AP reported.

None of the captured mosquitoes have carried disease yet, but they tend to be aggressive and target humans. If one happens to bite a person with dengue (a disease typically brought in from foreign countries), the disease will likely spread as it has in southern Florida, There have been about 200 cases of Dengue in California since 2010, according to The AP. Symptoms include headache, body pains, a rash similar to measles, and in some rare cases death.

"Vector control agents" will focus on controlling the spread of Aedes aegyptis. The California Department of Health has also issue warnings and trappings, the AP reported. 

"The nightmare scenarios is it gets established in California and then a mosquito bites someone with an imported case of dengue," said Tim Phillips of the Fresno Mosquito and Vector Control District.

It won't be easy.

"It's very difficult to control because of its biology," said Steve Mulligan, who is working on the Clovis outbreak. "They like humans and will come inside the house. We're trying hard to eliminate it."

Researchers studying its origins believe the mosquitoes come from Central America. Like most of their kind water is the key to their survival and reproduction, according to The AP. Health officials have gone door-to-door in high risk areas to warn residents about eliminating standing water.

"If it gets away it will change the way we live in California. You may not be able to sit on your patio and enjoy a cup of coffee during the day without getting bit," Phillips said.