Bartonella is a bacterial infection that often appears alongside Lyme. In the US there is no current evidence that ticks transmit it, and the question is still under investigation. The proven routes are cat scratches and bites, cat-flea feces, and body lice. It is hard to detect, which is why a thorough Lyme workup checks for it properly.

Babesia is a malaria-like parasite carried by the same tick that carries Lyme. Because it is a parasite and not a bacterium, the antibiotics typically used for Lyme have no effect on it. It is seldom tested for, so people can finish Lyme treatment, still feel sick, and not understand why.

The CDC classifies Washington DC, Maryland, and Virginia as high-incidence areas for Lyme. Standard two-tier blood testing catches only 22 to 36 percent of early infections, so a negative result does not rule Lyme out when symptoms are present. Free state and county programs identify a tick's species but do not test it for disease, and broader co-infection testing is not routinely ordered.