The single celled organism may cause disease but it is also essential to important life processes like decomposition

Bacteria are the single celled organisms, and are prokarotic. They cause numerous diseases like Tubeculosis, leprosy, lyme disease, cholera etc. At the same time, some of them are present in the environment and decompose the debris and add to the organic material of the earth. For example, Bacillus, Pseudomonas etc.