Fluorescence in situ hybridization (FISH) is a complementary molecular tool for the clinical diagnosis of infectious diseases by intracellular and fastidious bacteria that can be difficult to diagnose through standard antibody testing [111], and our chronically ill patient population demonstrated high co-infection rates for Babesia and Bartonella by antibody, with proof of active co-infections by PCR, VEGF, and FISH testing. Here, VEGFA is linked to coinfection.