Molecular-clock/phylogeographic analysis of HIV-1 group M sequences dates the founding cross-species transmission (SIVcpz to humans) to ~1920s Kinshasa (then Léopoldville, Belgian Congo), with subsequent spread along colonial-era rail and river trade routes driving the epidemic’s establishment through the 1960s — roughly four decades and a different continent-region of exposure before the disease was first clinically recognized as a novel syndrome (rare Pneumocystis pneumonia/Kaposi’s sarcoma clusters in US cities, 1981), via clinicians and public-health surveillance already primed to notice unusual disease in a specific population.
relevance_note: canonical primary example, external to the COVID case, that a pathogen’s first-ascertained human cluster can be separated from its true zoonotic spillover by decades and thousands of kilometers, purely as a function of where surveillance/clinical attention happened to be concentrated rather than where transmission began.