Fragen über die Beispielsätze mit, und die Definition und Benutzung von "Viruses"

A: It means “belongs to” in this sentence, i.e. “belongs to the category of unicellular microorganism”

A: I would say:

Viruses live off humans. They can’t survive on their own.

“Live off” means that they need something that a human has in order to live, such as nutrients

A: × Viruses continue evolving, and one that has advantageous genes survives into the next generation by natural selection.
✓ Viruses continue evolving, and one that has the most advantageous genes survives into the next generation by natural selection.

A: × Viruses don’t always replicate themselves accurately, they can mutate.
✓ Viruses don’t always replicate themselves accurately; they can mutate.

Punctuation is important for written work. If you were speaking this sentence, it would be perfect!