The article presents the results of electron microscopic studies of blood samples of cattle infected with monoinfections: bovine leukemia virus, foot-and-mouth disease virus and bovine ephemeral fever virus. Previously, the author identified nanometer-sized microorganisms in the blood of animals and humans with pronounced signs of gram-negative prokaryotic cells and characterized as nanobacteria. Nanobacteria at a concentration of 108-1010 cells/ml were detected by electron microscopy in the blood of animals infected with the virus of leukemia, foot-and-mouth disease and ephemeral fever. In addition, the technique, which includes the stages of freezing-thawing of blood samples and subsequent double centrifugation (500 and 2000 g), allows to identify cells of lymphocytes affected by nanobacteria, which gives grounds for concluding about associated effects on lymphocytes of nanobacteria and viral pathogens. It is quite obvious that the death of immunocompetent cells from nanobacteria, as well as their presence in high concentrations in the blood and organs of animals, loads the bodys immune system and thereby contributes to the development of secondary immunodeficiency with all its consequences