A small but growing number of athletes, from golfer Rory McIlroy to cyclist Tejay van Garderen, have canceled their trips to the Summer Olympics in Rio due to fears about the Zika epidemic.

But what are the chances that visitors and athletes could become infected?

“Almost zero,” Brazil’s new health minister, Ricardo Barros, said recently. He and other officials note there are far fewer mosquitoes active in August, when the Games are being held, because it’s winter in the Southern Hemisphere. Already, reports of new cases have plunged in Rio state — from 3,000 to 3,500 a week earlier in the year to just 30 cases a week in June, officials say. Meanwhile, 80 percent of the city’s buildings have been inspected for mosquito breeding sites, authorities maintain.

Doctors and scientists agree that the Zika threat is declining. They caution, however, that there may be more risk than the government is acknowledging. And some Brazilians are skeptical of the official progress reports at a time when the country’s government and economy are in crisis.

“I am seeing an optimism which is a little exaggerated,” said Jesse Alves, an infectious diseases specialist at the government-run Emilio Ribas hospital in Sao Paulo.

Since the Zika epidemic took off in Brazil’s northeast last year, it has been tied to 1,600 cases in this country of the birth defect microcephaly, which causes abnormally small heads and can result in learning and cognitive disorders. Zika is also linked to Guillain-Barré syndrome, a paralyzing nervous disorder.

In February, as Zika spread through the Americas, the World Health Organization designated the virus a public health emergency. The organization says pregnant women should not travel to Brazil. The warnings have spooked some athletes and tourists.

“People just aren’t comfortable going down there and putting themselves or their family at risk,” McIlroy told reporters last week.

Some 160,000 Brazilians have caught Zika this year alone. But reports of new suspected Zika cases have dropped 87 percent nationally from February to May following a publicity campaign urging people to eliminate breeding sites. The Aedes aegypti mosquito that transmits Zika lays its eggs near standing water, in places such as flower pots, buckets or old tires.

Few experts doubt there will be a further decline in cases in the next few months in Rio. Years of data on dengue, a disease also carried by the Aedes aegypti mosquito, show that there is typically a big drop-off in August and September.

“We will have less mosquitoes and less infected mosquitoes,” said Maria Sallum, a professor in the entomology department at the University of Sao Paulo. “I don’t know if there will be zero. I can’t affirm this.”

Laura Harrington, a professor of entomology at Cornell University, said mosquitoes slow down in lower temperatures, when the Zika virus takes much longer to go through its incubation period. Minimum temperatures in Rio in August are about 62 to 72 degrees Fahrenheit. But the days generally warm up to between 75 and 90 degrees. That’s enough for the insects to become active and bite.

“If there are Aedes aegypti out there flying around, then there is a risk,” Harrington said.

Some Brazilians have questioned the ability of the country’s interim government to respond to the Zika outbreak. A new cabinet and interim president were installed after President Dilma Rousseff was suspended in a controversial May 12 Senate impeachment vote.

Barros, the new health minister, has no medical degree. His arrival prompted the resignation of several top officials who disagreed with the change of government.

They include Claudio Maierovitch, who until recently was the top official in the department’s main branch dealing with transmissible diseases. He said that meetings between ministry technicians and regional health officials are less frequent under Barros and that decisions on the Zika crisis are not being made.

“I am very worried, and many other people are as well,” he said.

The new minister appeared unaware of how Brazil’s Zika data is gathered when discussing the epidemic with reporters in Rio recently. “We have a test that is done on everyone who presents symptoms,” Barros said. In fact, health officials in four states said laboratory tests for Zika are generally done only for pregnant women, children and victims who die. Everyone else showing symptoms of the disease, like rash and fever, are listed as “probable” cases.

A Health Ministry spokesman, who spoke on condition of anonymity under internal regulations, denied the political upheaval had hurt its response to the Zika outbreak, noting in an email that “each department has a highly specialized and qualified technical team” in addition to political appointees.

The Rio city government says it has dispatched 3,500 “health vigilance agents” — who are normally used to fight diseases such as dengue — to inspect buildings for potential mosquito breeding sites. Daniel Soranz, the city health secretary, said they had visited 5 million buildings this year.

But some residents — and even health workers — have questioned this claim. “It is impossible,” said Sandro Cesar, general secretary of Rio’s health vigilance agents’ union, which threatened a strike in February over a lack of insecticide, repellent and uniforms.

In interviews with three dozen residents in neighborhoods across Rio, The Washington Post found that fewer than half reported visits by the agents.

“They don’t go to poorer communities,” said Ezekiel Rodrigues, 27, from a low-income community in Taquara, a couple of miles from the Olympic Park.

Rio’s health secretariat maintains that the city has enough health vigilance agents for the work, despite not hiring additional employees. “What changes is the way work is organized, with more focus on field work,” said a spokeswoman, who spoke on condition of anonymity in line with internal policy.

The WHO recommends that people attending the Olympics use insect repellent and wear clothing that covers as much of their bodies as possible. Because Zika can be sexually transmitted, the organization is also urging travelers to practice safe sex or abstinence during their stay and for at least eight weeks after their return home. (For those whose wives or partners are pregnant — like Van Garderen, the American cyclist — the safe-sex recommendation is for the duration of the pregnancy). Brazilian authorities say they will distribute 450,000 condoms at the Olympics.

The WHO says it sees no reason to move the Games, arguing that a change in location would not significantly alter the international spread of Zika.

Arthur Caplan, a bioethicist at New York University, was among 230 academics and medical professionals from around the world who signed a letter calling for the Games to be moved or postponed. He said the Brazilian government should not depend on “best-case scenarios,” since even a small number of cases of Zika could result in tragedy.

“What if two or three babies are born with microcephaly afterwards? What will that do to the Olympic movement?” he said.