system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are there different types of blood?

On top of that, how can some people's blood be universal?
gpt
Evolutionarily speaking, diversity within the gene pool is good. Imagine a virus developing that is fatal to everyone with blood type A. Having B and O blood types allows the human race to survive.

As far as different blood types being universal, think of blood types as keys. Type B keys don't fit into Type A locks, nor vice versa. Type  AB locks can accept either key. Type O is like a master key that can fit in every lock.