A municipal worker stands next to a banner touting Carnival before spraying insecticide at Sambodrome in Rio de Janeiro. Municipal workers sprayed insecticide around Sambadrome, where the city's Carnival takes place every year, and where the archery competitions will take place during the Rio Olympics. The operation is part of the city's effort to prevent the spread of Zika's vector, the Aedes aegypti mosquito, according to a statement from the Municipal Health Secretary. (Pilar Olivares/Reuters)

Long before Usain Bolt lines up in the starting blocks or Katie Ledecky dives into a pool, a big Olympic race is already underway. Organizers for the Summer Games have a long checklist of tasks that still need to be addressed — from infrastructure to competition venues to budding health concerns — and precious little time to get it all done.

The long run-up to the Summer Games has been marred by monetary concerns, a scaled-back vision for the Olympics and some lingering doubts that Rio can have everything ready in time for the Opening Ceremony on Aug. 5. The torch will be lit in six months, and organizers can hear the clock ticking.

“What’s important is not the size of the challenge. It is the size and the strength of the response you give to this challenge,” said Mario Andrada, a spokesman for the Rio’s Olympic organizing committee.

Recent weeks have presented new challenges. Brazil is among the countries battling a breakout of the Zika virus. The World Health Organization has deemed the virus a global health emergency, and Olympic, government and health officials are on high alert. The virus is believed to have infected more than a million people in Brazil and has been linked to babies born with microcephaly, a rare birth defect.

Dead fish lie on the shore of Guanabara Bay in Rio de Janeiro, Brazil. Thousands of dead fish washed up on the shores of Rio's Guanabara Bay on Jan. 13, not far from where Olympic events will be held, environmental officials said. The incident was the latest involving water quality in the bay, where sailing, open water swimming and triathlon races are due to take place at the Games in August. (Ricardo Moraes/Reuters)

On Jan. 15, the U.S. Centers for Disease Control and Prevention warned that “pregnant women in any trimester should consider postponing travel to the areas where Zika virus transmission is ongoing.”

The International Olympic Committee sent a memo last week to countries participating in the Summer Games outlining some of the planned precautions, including the daily inspection of competition venues for standing water where mosquitoes might breed. Government and Olympic officials have stressed that the Olympics will take place during Brazil’s winter, a low season for mosquitoes because of cooler, drier conditions.

“Whatever needs to be done for the safety of the athletes and the people that work in the Games, we’ll be ready to the face the cost,” Andrada said, “but that’s not the point at the moment. The point is making sure that we win the battle against the mosquito and making sure that the people already infected . . . get the best assistance possible.”

Asked for a contingency plan and future precautions, Andrada expressed confidence that authorities and visitors alike will follow health advisories, and that the virus will be better contained six months from now.

“You know, right now we’re not very good at Plan B’s,” he said. “We’d like the Plan A to work.”

Officials with the U.S. Olympic Committee have been monitoring the situation through the Centers for Disease Control and Prevention and are in regular contact with the IOC, the WHO and Rio officials.

“Additionally, we’re taking steps to ensure that our delegation and those affiliated with Team USA are aware of the CDC’s recommendations regarding travel to Brazil,” said Patrick Sandusky, a USOC spokesman.

While the USOC expects to send more than 500 athletes to compete in Rio, Americans typically make up a large percentage of Olympic travelers. Brazilian government officials estimate that 90,000 U.S. citizens traveled for the 2014 World Cup and expect perhaps twice as many Americans for the Games.

When they get there, Olympic tourists will see a scaled-back operation compared with some recent Summer Games. Government funding has been cut, and the vision for the Olympics has continually been tweaked and trimmed. Brazil’s economy is still struggling, unemployment is spiraling and inflation has topped 10 percent. The World Bank estimates the nation’s economy shrank 3.7 percent last year and could contract a further 2.5 percent this year. The Rio state government has reached such dire straits that some public hospitals closed for the Christmas holiday.

Olympic organizers have tried to cut costs wherever possible, replacing some permanent structures with tents, altering its ticketing structure, even removing televisions from athletes’ rooms. When reports circulated that athletes staying in the Olympic Village would have to do without air conditioning, Olympic officials quickly found the money to quell concerns.

This has become even more important following the Zika outbreak — Andrada said that athletes are advised to keep windows closed and the air conditioning on to avoid mosquito bites in the Olympic Village.

Some key transport works are still not completed — including a new metro line that will link Rio’s famous beach suburbs to the West Rio area where the Olympic park and venue are.

Although violence in Rio’s many favelas has been worsening over the last year as cracks increasingly show in a “pacification” program that installed armed police bases, government and Olympic officials insist that security will not be impacted. Rio will have a variety of police and security forces throughout the Summer Games, more than 85,000 officers in total.

“Rio will be the safest city in the world during the Games,” Andrada said. “So we have no worry with the security of the tourists, of the athletes and those who will work on the Games.”

Rio officials say with six months to go, construction at competition venues is 70 percent complete, and they’re confident everything will be done on time. Twenty-two test events have been staged, and 23 more are planned for the next three months. Officials had to postpone the velodrome testing event last month by at least six weeks until the track is operable.

“We’d rather take the heat on having an event postponed and making sure that the real event and the Games will be perfect,” Andrada said.

Water pollution remains a concern. The former chief executive of World Sailing says he was forced out for trying to move the Olympic competition away from polluted Guanabara Bay after independent testing conducted by the Associated Press found disease-causing viruses linked to human sewage at levels thousands of times above what would be considered alarming in the United States or Europe. The tests include the sailing venue as well as those for rowing, canoeing, open-water swimming and triathlon.

In a recent BMX test event, competitors had safety concerns with the course, and officials scurried to improve the track in the hours before competition.

“The highway of the preparation of the Games is not straight and flat,” Andrada said. “It is not straight and flat for any city, and Rio is preparing itself in a normal way, and we learn from the mistakes we make.”

Maese reported from Washington.