Scientists understand (but the general public does not always understand) that all measurements have uncertainty. A false negative would conclude that you did not have a disease when in fact you did. A false positive would conclude that you had a disease when in truth you did not. Molecular tests to identify viruses have uncertainty. The test for Zika virus is relatively new and is good at detecting Zika virus. However, it is new and we cannot yet know if people who have the virus are always correctly identified. In other words, the rate of false negatives is uncertain; more data is needed. The Zika virus test can also detect related viruses that are not Zika. This gives false positives.
Agencies responsible for informing the public and the media try to give a simple answer rather than a complicated one. The uncertainty surrounding the number of infections in any disease is typically left out of the communication, because it would confuse the core message: the number of Zika cases. In deciding how to present numbers of cases, does the agency err on the side of false positives or false negatives?
The May 20,2016 Zika numbers from the CDC reflect this quandary. The CDC has decided that best practice would be to report the numbers of all pregnant women who have positive laboratory tests and considers the probability of false positives to be relatively small. Previously, Zika infection was only confirmed for pregnant women who had both reported symptoms consistent with Zika infection and had a positive test. However, Zika can have mild symptoms or some people may be asymptomatic. Relying on laboratory test data only is considered closer to the true number (few false positives), thus the new standard. By the new standard, the number of pregnant women considered to have Zika virus exposure is triple the previous number. The new numbers for the US states plus D.C. are 157 cases. The new numbers for US Territories (mostly Puerto Rico) are 122 cases. These are surely underestimates because not all pregnant women are tested for Zika.
Meanwhile, Congress is marching toward gridlock with competing, mutually unacceptable Zika funding bills in the House and Senate. Unfortunately, the Zika epidemic progresses with no regard to political gridlock.