What diseases were most effective during the westward expansion?

During the westward expansion, diseases such as smallpox, influenza, and measles had devastating effects on both settlers and Indigenous populations. Smallpox, in particular, was highly lethal and often spread rapidly among Native American communities who had no prior exposure or immunity. Additionally, cholera and dysentery became prevalent due to poor sanitation and crowded living conditions, further exacerbating the health crises of the time. These diseases contributed significantly to the mortality rates and social upheaval experienced during this period.