A rare brain-eating amoeba is responsible for at least 10 deaths in the Pakistani city of Karachi in recent months, health officials believe.
The source of the parasite is not yet known, but it is thought victims may have been exposed to it when using water to rinse their nasal passages.
The amoeba, Naegleria Fowleri, lives in warm water and kills its victims by destroying brain tissue.
Officials are now increasing the amount of chlorine in the public water supply.
The deaths are in various locations across Karachi, Pakistan's biggest city.
Dr Shakeel Mallick, who works for the provincial health department, said nine of those killed were men, while one victim was a child of four.
Officials suspect other cases may have gone undetected. Dr Mallick said hospitals were now being "vigilant".
The ministry was "very concerned" about the amoeba, he said.
Cities on alert
Although the amoeba is usually picked up in contaminated pools or lakes, only one of those killed had been swimming.
Officials are therefore concentrating their attention on the possibility that people picked it up when cleaning out their nostrils - a practice which is common in South Asia, BBC regional analyst Jill McGivering says.
The amoeba travels to the brain through the nasal passages.
Those infected have symptoms including fever, nausea and vomiting, as well as a stiff neck and headaches. Most die within a week.
The World Health Authority's Musa Khan says other cities across Pakistan have been put on alert.
An awareness campaign has also been launched among health workers and the public.
"People should avoid getting water too deep into their nostrils," Mr Khan said. "Those with symptoms should seek help immediately."
People are being advised to use boiled or chlorinated water to rinse their noses, and to clean out domestic water tanks where amoeba may flourish.
The amoeba cannot be passed from person to person.