What is a single celled organism that have no nucleus and can either benefit or harm the body

Bacteria

Explanation:

Bacteria have no true nucleus because it's a prokaryote. It couldn't have been a virus because all naturally occurring viruses are pathogenic unless treated by radiation.

We have two types of bacteria,

Pathogenic bacteria will definitely cause disease. Commensals, however, have don't harm us and have the ability to help us.

Eg: Flora (bacteria) in our intestines produce vitamin K and most of the vitamin B complex. They also compete with the pathogenic bacteria that you might've ingested and don't allow them to increase to the number that can cause disease.

age range would be the answer