Smallpox for example and Polio (almost)
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One factor is the rate at which the virus can mutate. Another is the number of species that can be affected.
So for instance, flu mutates quickly and it crosses species. So because it can infect so many different species it has a lot of opportunities to mutate. That makes it almost impossible to develop a universal flu vaccine.
Fortunately, flu is mostly season. So in the US, we look for the flu strains prominent in Australia earlier in the year before the US flu season starts. They make a vaccine that prevents the Australian version and other similar versions. Sometimes the virus mutates enough between the Australian flu season and the US flu season that the vaccine that was developed for the US isn’t as effective.
Australia tends to look at flu in southeastern Asia to develop their vaccine.
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