Residents in Sydney's south-west are being urged to slap on some mozzie repellent with a virus detected in mosquitos around the Georges River.
The infection, known as the Barmah Forest virus, is transmitted to humans through mosquito bites.
It has been detected in mosquitoes around the river by the NSW Arbovirus Surveillance and Mosquito Monitoring Program.
A large number of saltmarsh mosquitoes have been found near the Deepwater Park area of the Georges River.
Symptoms of the virus include fever, joint pain, rash and tiredness or weakness, with symptoms developing within seven to 10 days of being bitten.
These symptoms can last for several days, although some people may experience them for weeks or months.
There have been no reports of the virus locally, but the situation is still being monitored.
Advice for the public is to avoid being bitten as there is no specific treatment for the virus.