Workers wearing fumigation masks traveled methodically through high-rise public housing estates in seven separate areas of the island, inspecting plant pots closely as they sprayed insecticide via thermal fogging machines.

The health ministry on Saturday confirmed Singapore’s first locally-transmitted case of Zika, with the tally rising to 41 just a day later. All of the infected people were either residents of the Aljunied district or workers at a construction site owned by GuocoLand in the area.

See also: Hong Kong Confirms First Case of Zika Virus

“We expect to identify more positive cases,” the ministry said on Monday in its latest update on the outbreak.

“Given that the majority of Zika cases are asymptomatic or mildly symptomatic, and mosquitoes in the affected areas may already have been infected, isolation of positive cases may have limited effect to managing the spread,” it added.

Singapore, a major regional financial center and busy transit hub, which maintains a constant vigil against the mosquito-borne dengue virus, reported its first case of the Zika virus in May, brought in by a middle-aged man who had been to Brazil.

GuocoLand, which is headquartered in Singapore and has developments across Asia, was ordered on Saturday to stop work on the building site where 36 of the infected people worked. It will remain closed until the company rectifies the conditions that allowed mosquitoes to breed and steps up preventative measures, the health ministry said.

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The Zika virus, carried by mosquitoes, was detected in Brazil last year and has since spread across the Americas. It poses a risk to pregnant women because it can cause severe birth defects. It has been linked in Brazil to more than 1,600 cases of microcephaly, where babies are born with abnormally small heads and brains.

The World Health Organization said on Sunday it did not know “which lineage of Zika is circulating” in Singapore or “what the level of population immunity is to this lineage of Zika in Asia.”

Singapore’s health ministry said some 19 mosquito breeding habitats were detected and destroyed in its first sweep of the Aljunied area on Sunday when it fumigated around a third of the 6,000 homes.

“I feel afraid,” said Ng Kai Yee, an 18-year-old female student who lives near the construction site believed to be a source of the outbreak. “I heard quite a lot about how harmful Zika virus is to girls, especially pregnant women.”

Authorities have urged those living and working in the risk areas, especially pregnant women, to monitor their health and seek medical attention if they are unwell.

Of the 41 people known to have been infected, 34 have already fully recovered. Only the first case reported was a woman.