Those who believe that Americans are far-removed from the threat of Zika are in for a wake-up call. Puerto Rican blood banks have seen a steady rise in the number of Zika-infected blood units collected.

The blood contamination is the clearest sign of how quickly the disease is spreading in the area, Centers for Disease Control and Prevention Director Tom Frieden told reporters Friday.

"If current trends continue, thousands of pregnant women will get infected with Zika," said Frieden. "And there could be between dozens and hundreds of children with microcephaly born [there] in the next year." Currently, Puerto Rican women give birth to about 32,000 babies a year, according to STAT.

The blood bank data reveals that 1.1 percent of donated blood was infected, meaning that approximately 2 percent of Puerto Rican adults are contracting the virus monthly -- and mosquito season hasn't even reached its summer peak yet. Frieden estimated that this translates to an annual infection rate of about 25 percent for Puerto Rico's 3.5 million residents.

This news from Puerto Rico is a sure sign that Zika is slowly-but-surely prowling towards more of the United States. CDC officials said on Thursday that the number of pregnant women infected with Zika in the continental U.S. has risen to 234. Of those who have given birth, they cited six cases with abnormalities -- three of which died before birth but still showed evidence of defects.

If the Zika virus does penetrate more of the country, HealthGrove found the 10 cities that would likely be most afflicted.

Using data from a recent study in the Public Library of Science, HeathGrove considered five factors that would warrant a high risk for a Zika epidemic:

- Counties with recent local Dengue and/or Chikungunya transmission
- Potential abundance of Aedes aegypti mosquitoes (known to carry the virus) in January and July, classified as None, Low, Moderate, or High
- Estimated monthly average arrivals to the U.S. from countries on the CDC Zika travel advisory
- Population

HealthGrove then weighed all of these factors (in the order listed above) to calculate an overall "risk level" of a Zika epidemic in each city, scored out of 10. The city with the highest risk level had the score closest to 10.

When studying the results, HealthGrove found that the biggest risk areas in the United States are in the South and Southeast -- particularly Florida and its surrounding states. These warm, humid climates are ideal for mosquitoes to thrive. They also receive many visitors from South and Central America (where many countries plagued with Zika are found) due to their relative proximity.

The risk levels were rounded to the nearest hundredth place, though their ranking reflects each city's unrounded score. In the case of a true tie, the tie goes to the city with a higher population.

Risk level: 8

Mosquito level in January: None
Mosquito level in July: High
Number of people traveling to the U.S. from Zika countries: <10,000
Population: 727,689

Risk level: 8.01

Mosquito level in January: None
Mosquito level in July: High
Number of people traveling to the U.S. from Zika countries: <10,000
Population: 1,251,849

Risk level: 8.13

Mosquito level in January: None
Mosquito level in July: Moderate
Number of people traveling to the U.S. from Zika countries: 500,000-1,000,000
Population: 5,614,323

Risk level: 8.14

Mosquito level in January: None
Mosquito level in July: Moderate
Number of people traveling to the U.S. from Zika countries: 500,000-1,000,000
Population: 6,490,180

Risk level: 8.38

Mosquito level in January: Low
Mosquito level in July: High
Number of people traveling to the U.S. from Zika countries: <10,000
Population: 1,419,127

Risk level: 8.49

Mosquito level in January: None
Mosquito level in July: Moderate
Number of people traveling to the U.S. from Zika countries: 1,000,000-2,000,000
Population: 20,092,883

Risk level: 8.86

Mosquito level in January: Low
Mosquito level in July: Moderate
Number of people traveling to the U.S. from Zika countries: <10,000
Population: 420,392

Risk level: 9.14

Mosquito level in January: Low
Mosquito level in July: High
Number of people traveling to the U.S. from Zika countries: <10,000
Population: 2,915,582

Risk level: 9.43

Mosquito level in January: Low
Mosquito level in July: High
Number of people traveling to the U.S. from Zika countries: 500,000-1,000,000
Population: 2,321,418

Risk level: 10

Mosquito level in January: Moderate
Mosquito level in July: High
Number of people traveling to the U.S. from Zika countries: 1,000,000-2,000,000
Population: 5,929,819