Bacteria are single-celled, microscopic organisms that can be found practically anywhere. Bacteria can be found in any climate and geographical area on the planet. Some live in the air, while others live in the water or the earth. Bacteria can be found on and within plants, animals, and humans. Although the word "bacteria" has a negative connotation, bacteria play an important role in both organisms and the environment. A bacterial infection occurs when a hazardous strain of bacteria multiplies on or within the body. Bacteria have the ability to infect any part of the body. The signs and symptoms of a bacterial or viral infection differ depending on whatever part of the body is infected. The symptoms of both might be very similar at times.