In persons who have had hemorrhagic fever with renal syndrome (HFRS), destructive and functional disorders of the tubular apparatus of the kidneys persist for a long time, which can create prerequisites for the addition of a secondary infection and the development of pyelonephritis. Separately, the authors consider the intestinal microflora one of the possible sources of kidney infection, which is confirmed by the frequent detection of bactercuria in intestinal dysbiosis and the presence of dysbiosis in patients with pyelonephritis.