Cell

You have to be more specific with your question. I understand that you imply that unicellular organisms are primitive, compared to multicellular organisms, and thus have fewer cells.

That is a common miss conception about biology. Actual simple cellular organisms and olygocellular organisms are neither primitive nor simple.

In order to survive 3.6 billion years since the estimated origin of life they had been evolving the same ways as multicellular organisms. They had changed and adapted threw a long period of trial and error system. As well as we share a primitive ancestor with modern primates, all modern bacteria share a common primitive ancestor that is completely different to the actual species known.

Modern bacteria are nasty, strong, well built machines designed to survive extreme conditions, and thrive on the must hostile enviroment. We had adapted to defend to bacterias as fast as they had adapted to keep infecting us.

I expect this helps you. If you aremore specific i will gladily help.