SANDANSKI, Bulgaria—Like many of his Greek compatriots, Evangelos Emmanouil voted in Sunday’s referendum. But after casting his ballot in the city of Serres in the morning, he drove about an hour north, across the border to his home in Bulgaria.

Mr. Emmanouil lives and works as an accountant in Sandanski. Though not in Greece, the town isn’t a good place to be these days, he said.

“Bulgarian cities along the border have deep links with Greece, and they are now suffering along with the people on the other side,” said Mr. Emmanouil, who as a Greek citizen was eligible to vote in the referendum.

The escalation of the Greek debt crisis has sparked fears of contagion in neighboring countries and across Europe. Eastern European central banks in recent weeks have rushed to ease concerns among investors and depositors that the Greek crisis could infect their fragile economies.

But in Sandanski, contagion is already real. Famed for its thermal springs, this town of 30,000 people caters to Greek visitors. Most shops boast bilingual signs, and local restaurants and bars are filled with the sound of Greek music.