Horses originally evolved in the Americas, crossed the land bridge to Asia, and thrived. Somewhere along the line, every horse in the Americas died. I don’t think the cause was hunting to extinction. Lethal bugs could be a major factor in evolution and species extinction. The biggest threat to civilization is not terrorist nukes but bio gone wrong...either human-created in the lab or by natural means.
Bioweapons have strict limitations. Influenza is the biggest bio threat around. Nothing else comes close. Every other bio agent falls by the wayside for several reasons:
1) Must be pulmonary, spread by cough and sneezing. And must cause coughing and sneezing. Must exist in quantity in the upper respiratory tract and sinuses.
2) Must mutate frequently to avoid both immunity and declining mortality.
3) Must have significant animal vectors.
4) Must have an incubation period between 4-21 days.
5) Must not incapacitate or show symptoms during much of incubation period.
No other virus or bacteria can match influenza. The closest is the common cold, though far from typically lethal. It does so because there are seven major different pathogens that can cause colds, and one of them has over 100 variations. They have to all gang up, just to match the infection capability of influenza.