Many antibiotics originated from other bacteria and from fungi. Bacteria have been exposed to other bacteria and fungi for up to several billion years. Bacterium has waxy coat, can survive long periods in air and house dust. Prolonged use of one drug is optimal situation to select for resistant strain. Once resistant strain becomes widespread, that antibiotic no longer effective.
Humans are accidental host.
Recovery may take months. Sporeformer, widely found in soils. But obligate anaerobe, only grows when oxygen not available. Viruses are found in all 5 Kingdoms. In either case, can take over a host cell.