There are tons of people whose immune systems are strong enough to naturally suppress nearly any kind of infection. The stories a doctor serving in the rural areas of Sierra Leone told literally shocked me speechless.
In countries with poor health facilities, naturally resistant people tend to be a significantly higher percentage of the population than in the west, possibly because a lot of those with weaker immune systems (who would otherwise be alive in richer countries) die early.
This might also be an explanation of why covid has claimed so few lives (relatively) in the poorer parts of the world (though another explanation could be that those parts of the world have less air traffic with the rich world).