Syphilis in the city is on the rise, and health officials aren’t sure why.

Instances of the chancre-causing disease spiked 8 percent in the first half of 2014, from 585 to 628 cases.

The disease has shown a steady increase long-term as well with 1,167 reported cases in 2013, compared with 117 cases in 2000.

Chelsea is the most hard-hit area. The most recent statistics showed an infection of nearly six times the citywide average.

Officials are scratching their heads, since studies show condom use is remaining steady.

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