The term INFECTION-CAUSING BACTERIUM can be summarized as a microorganism that causes an infection. A GERM, which is a four letter word for this term, is a general term for any microscopic organism, such as bacteria, viruses, fungi, and parasites, that can cause disease. Bacteria are single-celled organisms that can exist either as independent organisms or in symbiotic relationships with other organisms. Bacteria can cause infections when they enter the body and multiply, leading to inflammation and tissue damage.