Introduction to the study of microbiology. Microorganisms: generalities. Prokaryotes vs eukaryotes. Size, shape and general organization of bacteria. Bacterial cell surface structures: cytoplasmic membrane; bacterial cell wall of gram-negative and gram-positive; Wall of alcohol acid resistant bacteria. Internal structures: the cytoplasm, nucleoid, plasmids and transposons, ribosomes, endospores. external structures to the bacterial wall: Capsule, glycocalyx, flagella, pili and fimbriae. Binary fission and horizontal transfer of genetic information: transformation, conjugation, phage transduction (restricted and generalized), phage conversion. Bacterial pathogenicity. Factors that promote host colonization: mobility, adhesion, invasion, competition for iron and other nutrients. Factors that promote innate and adaptive immune responses. Virulence factors that cause damage to the host: ability to induce the production of inflammatory cytokines and chemokines, bacterial endotoxins and exotoxins. Antibiotics and chemotherapics. Direct and indirect methods of diagnosis: isolation of bacteria of veterinary interest, cultural characters, dyeing characters, biochemical and serological tests, immunofluorescence, ELISA, molecular diagnosis of bacterial infection. General characteristics of viruses. Size and shape. The structure of the virus. Criteria for classification of viruses. Viroids and prions. Life cycle of viruses of veterinary interest. Oncogenic viruses. Lytic cycle and lysogenic cycle of bacteriophages. Viral Pathogenesis. Antivirals. Types of viral infection. Isolation of viruses and CPE. Direct and indirect methods of virological diagnosis: PCR, ELISA, Immunofluorescence.