Why retard medical science? Suppose you get earlier exposure to malaria, Ebola, or HIV? Say 1860, Lenoir's IC engine is used in a powered trike, cars & airships are created, & exploration into Africa, CAm, SEA happens, where Europeans meet diseases they can't cope with. In response, science figures out ways to kill viri (or boost the immune system, or both...). This could also improve CBW, which militaries might like... And you conceivably butterfly away nukes as needless. OTOH, you make minor powers much more a threat, since lo$ CBW's much easier than nukes...