60 million buffs in 1873.
In 1873 you set up camp and waited for the buffalo to come to you. By 1877 you had to travel to find them, sometimes long distances. Other maladies were probably common in the herds as well.
From an interview with a former buffalo hunter in the early 1900s. He also stated that most buffalo hunters went broke in their first year out. Very tough industry.
This was a period when many settlers were traveling west with horses, oxen and other livestock from various areas of the east coat. Probably introduced some new diseases as well. Also see my Comment #56.