World Cup has copious amount of hookup culture (just look through Tinder when the season plays in your city), Grindr and general clubbing in these areas.

I've been a visitor during a World Cup and its not just the football people go for.

There's even a term for it: World Cup fever, not just isolated to world cups, many other sporting events have the same problem of being super spreading events for STIs https://sexualhealthbucks.nhs.uk/latest/world-cup-fever/

I'd also say prolonged contact and mingling is worse then a short one like above, plenty of time to pub and club crawl, sleep with multiple partners, allowing disease to expell after its normal 5-6day incubation period.