Ruchusatsawat K, et al. - Researchers undertook this observational study to appraise the long-term circulation of Zika virus in Thailand. Data from individuals with suspected Zika virus infection who presented at hospitals throughout the country and had biological samples (serum, plasma, or urine) tested for confirmation with PCR at the National Institute of Health laboratories in Bangkok were analyzed. From 1717 symptomatic individuals, 3089 samples were tested between January 2016 and December 2017. Zika virus infection was confirmed in 368 of these. From 29 of the 76 Thai provinces, cases of Zika virus infection were reported throughout the year. They identified higher odds of testing positive for Zika virus infection among individuals who came from the same district and were sick within the same year of a person with a confirmed infection compared to the odds of testing positive anywhere, consistent with focal transmission. The evidence reveals circulation of Zika virus at a low but sustained level for at least 16 years, suggesting the possibility of adaptation in Zika virus to persistent endemic transmission. This highlights the necessity for adaptions in health systems to cope with regular occurrences of the severe complications associated with infection.