The unglamorous truth is that the most common traveler ailment everywhere is stomach trouble, usually from food handling rather than water: undercooked dishes, buffet food sitting warm, raw salads, and pre-cut fruit. The standard defenses are eating food cooked fresh and hot, favoring busy places with high turnover, and carrying oral rehydration salts and a basic stomach kit so a rough day stays just a rough day.
Mosquito-borne diseases are the second tier in tropical and subtropical regions: dengue and chikungunya in cities, transmitted by day-biting mosquitoes, and malaria in specific rural regions, so repellent, sensible clothing, and prophylaxis where advised genuinely matter. Beyond those, think about rabies exposure from street dogs and monkeys, meaning don't pet, and seek care immediately after any bite or scratch; heat exhaustion and sunburn in hot climates; road accidents, statistically the biggest real danger to travelers, especially on rented scooters; and respiratory infections from air pollution in some megacities.