It depends. It’s hard to model and even think about the selection pressures accurately. West Nile virus showed a marked increase in virulence in corvids recentl

It depends. It’s hard to model and even think about the selection pressures accurately. West Nile virus showed a marked increase in virulence in corvids recently as did rabbit hemorrhagic disease. And let’s not forget about Mareks disease, where a non-sterilizing vaccine is thought to have contributed to a massive increase in virulence, killing nearly 100% of unvaccinated chickens now vs causing mild disease decades ago.