Final answer:
Obligate intracellular pathogens are bacteria that can only replicate inside the body of their host, relying on the host's cellular resources for reproduction and often evolving strategies to evade the immune system.
Step-by-step explanation:
The term for bacteria that can only replicate inside the body of their host is obligate intracellular pathogens. These bacteria require host cells to provide the necessary nutrients and environment for their reproduction. For example, among the Alphaproteobacteria, there are rickettsias, which are obligate intracellular pathogens that depend wholly on the host's cellular mechanisms to replicate as they cannot synthesize their own adenosine triphosphate (ATP). Similar to how viruses infect host cells and use the cell's resources to reproduce, obligate intracellular bacteria must invade host tissue and often develop strategies to evade the host's immune system, like producing protective layers or infecting phagocytic cells meant to destroy them.