Bacteria are small unicellular organisms which are also known as prokaryotes. As the Merck Manual explains, there are many different kinds of bacteria, and they can live in a multitude of different environments. Bacteria are different than viruses in that they have fully functional cells, which means that bacteria are able to survive and reproduce on their own. Although bacteria commonly form colonies, they are unicellular, which means that each cell is distinct and able to function on its own. Bacteria cause infections when they begin to reproduce within the human body and cause damage.
Routes of Infection
Many bacteria live in the human body and cause no problems. In fact, as My Optum Health notes, some of these bacteria (such as those that live in the intestines) are actually beneficial for the body. These bacteria only cause infections when there are too many of them (also known as bacterial overgrowth) or when they spread to other parts of the body (often by getting into the bloodstream or when there is an open wound). Other bacteria cause tissue damage and destruction no matter where in the body they are located.
Bacteria can cause infections by initiating damage in nearby healthy cells, causing them to die. In some cases, the bacteria will use the recently killed cells for fuel. Bacteria can also produce a sticky substance called a biofilm, which traps food particles for the bacteria to use. Bacteria can also secrete toxins called endotoxins, which poison nearby tissue. When bacteria get into the bloodstream, they can cause a condition known as sepsis. Not only does this trigger a potentially dangerous immune system reaction, it also allows the bacteria to gain access to tissues (such as the heart) that can be easily damaged by the bacteria.