BANGKOK (AP) — Thailand is being challenged by an infection cluster among migrant workers after keeping the coronavirus largely in check for most of the year. The surge of cases in a province near Bangkok threatens to undo months of containment efforts. The prime minister went on television to stress the need for vigilance to counter the local outbreak as well as resurgences in other countries. Prime Minister Prayuth Chan-ocha said Thailand must tread carefully as it relaxes rules for admitting foreign visitors. That step is crucial to helping the battered economy and reviving tourism that dried up when Thailand closed to regular passenger flights from overseas in early April.