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Simple Summary: A disease outbreak in a Swedish herd of beef cattle which may be related to a tick-borne infection initiated an in-depth study to investigate the presence of bacteria and viruses in clinically healthy and diseased cattle, as well as in wild deer and boars from the region and in ticks collected from the cattle and deer. The results indicated unusual types of Mycoplasma as possible causative factors, probably in combination with immune suppression due to granulocytic anaplasmosis caused by Anaplasma phagocytophilum. The latter bacterium was widespread in the herd and was also found in wild deer in the region, and in ticks collected from the cattle and deer. Genetic comparisons indicated that Anaplasma phagocytophilum was circulating among the cattle population, while circulation between cattle and deer occurred infrequently. An outbreak of disease in a Swedish beef cattle herd initiated an in-depth study to investigate the presence of bacteria and viruses in the blood of clinically healthy (n = 10) and clinically diseased cattle (n = 20) using whole-genome shotgun sequencing (WGSS). The occurrence of infectious agents was also investigated in ticks found attached to healthy cattle (n = 61) and wild deer (n = 23), and in spleen samples from wild deer (n = 30) and wild boars (n = 10). Moreover, blood samples from 84 clinically healthy young stock were analysed for antibodies against Anaplasma phagocytophilum and Babesia divergens. The WGSS revealed the presence of at least three distinct Mycoplasma variants that were most closely related to Mycoplasma wenyonii. Two of these were very similar to a divergent M. wenyonii variant previously only detected in Mexico. These variants tended to be more common in the diseased cattle than in the healthy cattle but were not detected in the ticks or wild animals. The DNA of A. phagocytophilum was detected in similar proportions in diseased (33%) and healthy (40%) cattle, while 70% of the deer, 8% of ticks collected from the cattle and 19% of the ticks collected from deer were positive. Almost all the isolates from the cattle, deer and ticks belonged to Ecotype 1. Based on sequencing of the groEL-gene, most isolates of A. phagocytophilum from cattle were similar and belonged to a different cluster than the isolates from wild deer. Antibodies against A. phagocytophilum were detected in all the analysed samples. In conclusion, uncommon variants of Mycoplasma were detected, probably associated with the disease outbreak in combination with immune suppression due to granulocytic anaplasmosis. Moreover, A. phagocytophilum was found to be circulating within this cattle population, while circulation between cattle and deer occurred infrequently. [ABSTRACT FROM AUTHOR]