- Large populations, it's no secret larger populations have more people who might be infected with Covid-19.
- Dense urban centers. Sal Paulo, Islamabad, Pekanbaru, Mumbai... these are cities I've been to in those countries where people live in close quarters, and not all have electricity and clean running water, in any of those cities. It's a recipe for a virus to spread.
- General wealth of the country. This is a stand in for all things medical, because in short, a more wealthy country is going to be able to mobilize doctors, nurses, ventilators, medications, etc. better than a poorer country. Again, Brazil has a strong chance here, and frankly India and Indonesia do too, but not like the USA has.
The big upside those four countries do have is younger populations. Covid-19 is affecting older people far worse overall than younger people. Sure, it's still killing people in their 20s and 30s, but it's far more deadly to people in their 60s, 70s and 80s. An upside for Indonesia and parts of Pakistan and Brazil are that islands and very remote areas make frequently contact with urban centers not too common. In other words, being a little remote in 2020 will turn out to be a good thing.
In short, I fear for, and pray for, my friends and acquaintances in those countries, because their risks are so high. If countries are going to get herd immunity, it's going to be countries like those first before the wealthy countries like the USA. Oh my heart breaks thinking of the people that need to work every day or they starve!