They are particularly worried that the disease is wreaking havoc in a region where the population has not encountered it before and that climate change may be allowing viruses like Zika to thrive in new domains.

The Brazilian government has stopped short of officially advising women not to get pregnant, but confusion and fear are spreading along with the virus.

“The situation is incredibly frightening,” said Andreza Mireli Silva, 22, a worker in a shoe factory in Sergipe State in northeast Brazil who is seven months pregnant. She said she was trying to avoid mosquito bites by wearing long pants despite the heat of the summer and applying insect repellent every three hours.

Zika, named for the forest in Uganda where scientists discovered it in the 1940s, often goes unnoticed in the people it infects and was not considered especially life-threatening before spreading to Brazil. But the advance of the virus here is focusing scrutiny on the resilience of a worrisome pest: Aedes aegypti, the mosquito that carries Zika and other diseases, including yellow fever and chikungunya.

“Brazil offers the ideal conditions for Zika to spread so quickly,” said Ana Maria Bispo de Filippis, a leader of the research team that has linked Zika to microcephaly. The country has “a susceptible population in which the majority of people never had contact with the disease.”

Before Zika’s arrival, Brazil was already grappling with a much deadlier epidemic of dengue, another virus transmitted by Aedes mosquitoes. Brazil had nearly 1.6 million cases of dengue in 2015, according to estimates from the Health Ministry, up from 569,000 in 2014. At least 839 people have died from dengue in Brazil this year, an 80 percent increase from the previous year. Some health officials say changes in weather and rainfall may be behind the surge.

Brazil waged war on the Aedes aegypti mosquito for decades during the 20th century before a vaccine was developed for yellow fever. Health agents deployed across the country to destroy habitats like water barrels and other open water sources where the mosquitoes thrive. The authorities even declared victory against the pest in 1955.