Research programme Co-existence of the host and parasites is a multidisciplinary programme. Different microorganisms coexists with different hosts and in this relation various responses are triggered in both. The final consequences of those relations are changes in life conditions of the host and the pathogenic agent. Studying individual microorganisms, their characteristics and circumstances in which they live and in which they become pathogenic for the host, represent an extensive multitude of events, which are named parasitic symbiosis. Research group Microbiology and immunology performed in the research period 1999 - 2003 extensive work studying these relations and the results are presented here in four separate parts: BACTERIOLOGICAL PART Monitoring and control of sensitivity of bacteria isolated from different contagious materials originating from daily diagnostics work, which are important for the Slovene pathology. Studying of the pathogenicity of bacteria: Helicobacter pylori, Neisseria gonorrhoeae, E. coli, Pseudomonas aeruginosa, against meticilline resistant bacterium S. aureus, Borrelia burgdorferi, C. pneumoniae was performed. VIROLOGICAL PART Studies of molecular ecology and epidemiology of hantaviruses, ricketsias and babesias were done. Examination of the genetic heterogeneity inside the individual type of hantaviruses directly isolated from the samples of natural hosts (forest and filed mice). For sorrow discrimination between the different pathogens, establishing of nucleotide sequence in PCR amplikons of different parts of the hantavirus genome was performed. Presence of noroviruses (intestinal Caliciviruses, from the Caliciviridae family ) in Slovenia was demonstrated. Suitable electron microscopy and molecular procedures were established for discovering viral pathogens. Research on the distribution of human herpes virus 6, which is very expanded in Slovenia (80% of the population older then 20 years demonstrated specific IgG antibodies in blood samples) was done. IMMUNOLOGICAL PART Studies of qualitative immune response against Borrelia burgdorferi s.l. and Borrelia afzelii antigens were performed. Dependence of the strength of the immune response from the antigenic structures of different Borrelia strains cultured in various experimental conditions was demonstrated. Mechanisms of membranous glomerulonephritis evolution, which depends on the creation of pathogenic immune complexes, which activate the complement, was further studied. Complement activation was connected with the induction of apoptosis. It was found that apoptosis represent one of the mechanisms, which contribute to the creation of membranous glomerulonephritis. Amount of inflammatory cytokines was measured in adult patients with sepsis and in very sick newborns. Involvement of cytokines in the experimental infection caused with bacteria Campylobacter jejuni and Lysteria monocytogenes and cytomegalovirus was studied. Established was, that the cytokines represent important factors in the pathogenesis of these disease. PARASITOLOGICAL PART With some improved and some newly introduced tests distribution and eventual danger of infection of people with Toxoplasma gondii (preferentially pregnant women and patients with eye disease), Cryptosporidium parvum, Blastocystis hominis was established. For the first time rare subtropical disease agents (Dirofilaria repens and Dermatobia homini) were described in Slovenia.