Air pollution: Kills about 7–10 million people per year. Avoid cold and dry seasons in places with bad pollution. Wear a mask or a scarf over your face.
Altitude sickness: More dangerous than most people think. Even if you don't notice severe symptoms it can do irreversible damage to your brain. Being fit and healthy doesn't help you avoid it. Be careful when going above 3000 m and give yourself enough time to get acclimatized.
Sun: You get sunburnt more easily on water, on sand, on snow, or in the mountains. Wear a hat/long sleeve/sunglasses/sunscreen.
Hygiene: wash your hands often and avoid touching your eyes or mouth. Don't neglect minor cuts on your skin in places with poor hygiene.
Food, water: Eat cooked food that is served hot. Eat fruits that you have washed and peeled yourself. Avoid meat and raw vegetables (including salads). Don't drink unsafe water. Avoid drinks with ice.
Mosquitos: transmit many diseases, especially malaria. Wear long pants, long sleeves, use mosquito net and repellent, avoid rainy season. Malaria pills can be bad for you, only use them if visiting high-risk areas. Of the different species of parasites causing malaria, P. falciparum causes the most severe form. Malaria-carrying Anopheles mosquitos only bite at night. Aedes mosquitos (dengue fever, yellow fever, chikungunya, zika) only bite during the day, dusk and dawn. Culex mosquitos (Japanese encephalitis, West Nile fever) bite in the evening and night.
Vaccinations: Yellow Fever vaccination certificate (lifelong validity) is required to visit some African countries. Many countries require it when arriving from a country at risk of Yellow Fever.
Other vaccines you might consider: hepatitis A and B (lifelong protection), tetanus (~10 year protection), typhoid (~5 year protection, ~70% effectiveness), rabies, seasonal flu, covid. "Routine vaccines"
Rabies: be careful around street dogs.
Snake bites: most dangerous of the "dangerous animals", especially in South Asia and Africa. Wear shoes and long pants when walking in the bush.
Ticks: check your body for ticks after visiting the woods in N. America and Europe (Lyme disease).