A university researcher thinks dating apps like Tinder and Grindr could be to blame for resurgence in syphilis.
Nearly 200 cases of the disease were reported in Auckland, twice as many as two years ago.
Auckland University researcher Peter Saxton says just because you've been cautious doesn't mean others have been too.
"One of the things that apps do, is even if you're not particularly sexually active yourself, you can meet someone who is."
Grindr markets itself as "the world's largest gay social network app", while Tinder caters for both straight and gay people.
Syphilis was thought to be wiped out in the 1950s, and if left untreated, can damage the heart and brain. It can also help other sexually transmitted infections like HIV spread more easily.
Nine in 10 cases were gay men, but heterosexual incidences of the disease rose 280 percent between 2014 and 2015.