Health authorities have confirmed the city's second case of Japanese encephalitis this year and are not ruling out the possibility of more instances over the summer.
Blood and sputum tests on a 35-year-old woman residing at Ching Nga Court on Yuen Long's Tai Shu Ha Road East tested positive for the virus on Saturday.
She is now in stable condition at Tuen Mun Hospital.
The woman reported feeling ill on July 4 after coming down with a fever, headaches and bouts of vomiting.
She did not travel out of Hong Kong in the two weeks before falling sick, but reported being bitten by mosquitoes in Yuen Long, the Centre for Health Protection said yesterday.
The woman visited a private doctor but was subsequently admitted to hospital after developing another fever 10 days later.
This is the second case of Japanese encephalitis to occur in the Yuen Long district this year.
A 26-year-old from Tin Shui Wai was infected with the virus last month.
The large bodies of water such as ponds and marshes abundant across Yuen Long are prime breeding grounds for culex mosquitoes, the genus of the pest that carries and transmits the virus.
Yuen Ming-chi, the Food and Environmental Hygiene Department's officer in charge of pest control, said one risk factor could have been the four pig farms located near the victim's home.
The mosquito-borne virus is often found in pigs and wild water birds.
"Mosquito prevention and extermination works have been deployed to the area in and around Ching Nga Court," he said.
Dr Chuang Shuk-kwan, the centre's consultant for community medicine, said the rainy season could bring about more cases.
"It is still summertime and there will still be [more] rainfall … there is a risk of further cases, but it is difficult to predict how many more," she said.
"Preventing and controlling the spread of Japanese encephalitis will require mosquito extermination work."
There were six reported cases of Japanese encephalitis in Hong Kong last year. Two of them involved people who contracted the disease locally, three had picked up the virus overseas, and in the sixth case the source is still unknown.
The two local cases reported last year involved residents of Tin Shui Wai.