With more than a thousand new cases of Zika reported in Puerto Rico for the first week of September, the numbers of those infected may rise even higher as the U.S. territory enters its fall rainy season.
Between Aug. 26 and Sept. 2, there were 1,024 new positive cases of Zika reported in Puerto Rico, according Candice Burns Hoffmann, a press officer with the National Center for Emerging and Zoonotic Infectious Diseases.
"These reflect just diagnosed cases," she said. "Because many people with Zika don't have symptoms, the number of people who have contacted Zika is likely much higher."
Outbreaks of mosquito-borne viruses like Zika tend to peak in the late summer and fall in Puerto Rico due to "hotter months with higher rainfall," Hoffmann said, adding that health officials fear that Zika will continue to spread and increase in the coming months.
With more than 16,000 cases already confirmed, Puerto Rico has had the highest number of reported cases of the other U.S. territories that have been affected by Zika.
The Zika carrier, the Aedes aegypti mosquitoes, like warmer weather and are also able to replicate the virus more efficiently at higher temperatures, she said. Increased rainfall can also be problematic because Aedes aegypti is known as a "container-breeding mosquito" because it likes to lay eggs in and around standing water.
"[There are] two wet times of the year, September through November, and a smaller rainy season in May," AccuWeather Meteorologist Jim Andrews said, adding that Puerto Rico sees an average of 73 inches of rainfall each year.
The average rainfall in May is around 8 inches, while September, October and November all have an average between 8 and 9 inches, he said. In addition to increased rainfall, temperatures between July and into October average around 82 F (28 C) , which further increase the breeding of the mosquito populations.
Despite recent tropical storm activity in the region, which can also increase breeding grounds for disease-carrying mosquitoes, Andrews said Puerto Rico rainfall has been slightly below normal during August and so far in September.
Slightly below-average precipitation has done little to slow what is becoming a major concern for health officials as the outbreak of Zika in Puerto Rico continues to spread.
"It's unknown at this time if Zika transmissions in Puerto Rico have reached its peak," Hoffmann said, adding that different parts of the island are also experiencing different rates of transmission, and not every location in Puerto Rico may peak at the same time.
"In addition to the 16,000 plus reported cases of Zika, there have been more than 1,000 pregnant women who have tested positive for Zika virus," she added. "This could result in hundreds of babies being born with microcephaly in the coming months."
The Aedes aegypti mosquito is also indigenous to Puerto Rico. These mosquitoes live in tropical, subtropical and in some temperate climates, making it even more difficult for health officials to control.
"It is very difficult to control or eliminate Aedes aegypti because they have adaptations to the environment that make them highly resilient," Hoffmann said. "It is likely that Aedes aegypti is continually responding or adapting to environmental change."
For example, it was recently found that Aedis aegypti is able to undergo immature development in broken or open septic tanks in Puerto Rico, resulting in the production of hundreds or thousands of mosquitoes every day.
In addition to combating natural adaptations, Hoffmann said they have also seen that there is a wide variety of resistance to some insecticides in Puerto Rico.
With more than 16,000 reported cases, less than 1 percent of the population of Puerto Rico has currently tested positive for Zika. However, the number could rise to 20 to 25 percent if the trend continues as it has.
In August, Puerto Rico's lead epidemiologist for the Centers for Disease Control and Prevention's Tyler Sharp told NPR that Zika is even more complicated than previous outbreaks of Dengue and Chikungunya, which were unable to be stopped by public health officials, because it is also transmitted through sexual activity.
"Since local transmission of Zika virus was first reported in Puerto Rico in December 2015, it has become widespread on the island," Hoffmann said. "Estimates suggest that if trends continue, at least one in four people, including women who become pregnant, may become infected with the Zika virus."