All living organisms are connected between each other because they have shared ancestor. I suggest you to read some stuff about evolution to make it more clear to understand.
Answer: natural selection
Parasite: Parasites are different from predators because parasites only take resources from one host, whereas predators eat many prey. A good example of this is the trematode parasite Schistosoma mansoni. An adult schistosome parasite lives inside of just one human host
