Human Endogenous Retroviruses (HERVs) are ancient sequences that became integrated into our DNA during evolutionary processes. The significance of their presence in DNA is still being studied, but HERVs appear to be involved in the regulation of the immune response. The gut microbiota is the set of microorganisms that physiologically colonize the gastrointestinal tract. Progressively increasing attention is being paid to the gut microbiota and its potential role in diseases of various kinds. In particular, it has recently been shown how abnormalities in the microbiota can affect immune regulation by determining the occurrence of certain diseases. One objective of the study is to evaluate whether there is an association between the expression of HERVs and clinical manifestations in pediatric age in patients with various diseases such as Kawasaki Disease, IgA Vasculitis/Schonlein-Henoch's Purpura, SARS-CoV-2 infection, or, finally, febrile virosis. Another objective of the study is to evaluate whether there is an association between the presence of particular types of bacteria in the gut microbiota and clinical manifestations in pediatric age in patients with Kawasaki Disease, IgA Vasculitis/Schonlein-Henoch purpura, SARS-CoV-2 infection or febrile virosis. The results from this may open new perspectives for both the care and therapeutic management of children with these diseases.