Question

why single cell organisms are considered immoortile

Answer 1

Actually, single celled organisms are considered to be biologically immortal. This is because they don't die as they grow old.

They usually undergo Mitosis (asexual reproduction) to reproduce, in which the organism itself gets divided into two (cell division).

This technically means that the same organism keeps getting split into new young ones (daughter cells). So there is no way we could say that the organism has died.

However biologically immortal does not mean that it can never die. The organism may die due to some other reason (external factors like injury,etc,.).