What type of small organisms are transmitted by the bites of fleas, lice, mites, and ticks?

Rickettsia are small, obligate intracellular bacteria that are transmitted primarily through the bites of arthropods such as fleas, lice, mites, and ticks. These organisms are well known for causing a variety of diseases, collectively referred to as rickettsial diseases, which include typhus and Rocky Mountain spotted fever. Rickettsia are unique in that they require a host cell to replicate, and their lifecycle is closely tied to the specific vectors that transmit them, which directly relates to their transmission method.

In contrast, protozoa are single-celled eukaryotic organisms that may also be transmitted through vectors, but they belong to a different classification than bacteria. Bacteria, while they can be transmitted through various means, are a broader category and do not specifically relate to the vector transmission pattern indicated in the question. Viruses are smaller infectious agents that require a host to replicate, but they represent another distinct type of organism with varied transmission routes. The specificity of Rickettsia to be transmitted through the mentioned arthropods makes it the correct choice for this question.