Lyme disease is not caused by a parasite. In fact, in North America, it is caused by a bacterium called Borrelia burgdorferi. Other species of Borrelia are also known to cause Lyme disease in other part of the world. As such, this disease can also be called Lyme Borreliosis. Bacteria are very different from parasites, as they do are not the same type of cell. As such, bacteria are known as prokaryotes, a primitive single-cell type lacking any membrane-bound organelles, such as the nucleus, the structure containing the DNA, and the mitochondria, the organelle responsible for the production of energy. On the opposite side, parasites are known as eukaryotes, a type of cells including many organelles surrounded by membranes. As a reference, animals, plants and human are also composed of eukaryotic cells.
The bacteria responsible for Lyme disease are transmitted by the bite of the blacklegged tick. However, this tick is also responsible for the transmission of other microorganisms, such as the protozoan parasites Babesia responsible for a disease called babesiosis. It is noteworthy that, as taxonomy of parasites is evolving quickly because of the new DNA sequencing technologies, this parasite is now officially called Theileria. However, this novel classification is not yet used in the medical field where this parasite is still known as Babesia.
This is where it becomes a little bit confusing. It is perfectly possible to get co-infected with two or more microorganisms when bitten by a blacklegged tick and this co-infection risk varied greatly depending on the geographical region. As such, between 2% and 40% of the patients having Lyme disease are susceptible to have babesiosis at the same time. This type of co-infection is one of the most frequent happening when bitten by a blacklegged tick. However, it is very important to know when a patient is co-infected by both microorganisms because these two illnesses are caused by two different types of microorganisms and do not therefore have the same treatment. Consequently, if someone has been formally diagnosed with Lyme disease and is not getting better or is getting worse with the appropriate treatment, it is very likely that it also have babesiosis. It is also possible, although less frequent, to have a co-infection with Borrelia burgdorferi and another bacteria or a virus. However, in the case of the co-infection with another bacteria, the treatment will be the same than the one for Lyme disease and the condition of the infected person will improve.
Furthermore, Lyme disease and babesiosis can have similar symptoms and are both very difficult to diagnose. As such, the formal diagnosis can be very delayed or they can be misdiagnosed as one another. In conclusion, infection with Babesia are not causing Lyme disease, but cause a illness with similar symptoms that is likely to be misdiagnosed as Lyme disease. Moreover, it is also possible to have Lyme disease and babesiosis at the same time.
Source: Centers for Disease Control and Prevention