MANILA—HIV is spreading faster in the Philippines than anywhere in the world.

The estimated number of new infections in this Southeast Asian nation has more than doubled from 2013 through 2018, reaching epidemic proportions among young men, according to the United Nations’ AIDS agency. The surge in the virus that causes AIDS stands out against the decline in estimated world-wide infections.

Determining the cause can be difficult and inevitably involves some guesswork among epidemiologists seeking answers.

But health advocates think a major driver has been dating apps, which have encouraged promiscuity in a country where traditional social norms have kept people from using contraception and learning about responsible sexual practice.

More than three-quarters of the 67,395 people diagnosed HIV positive in the Philippines between January 1984—when the first infection surfaced in the country—through May this year were diagnosed in the past five years.