The Greek word bakterion means stick or rod. Bacteria are the simplest microscopic organisms in nature that are single-celled without chlorophyll and have complex cell walls that cause disease in plants and animals. Bacteria are called natural scavengers. The branch of biology which deals with the structure, habitat, pathology, reproduction etc. of bacteria is called bacteriology. Anthony van Leeuwen Hooke is called the father of bacteriology and protozoology.