Good doctors could kill them
Disease was rampant, and doctors were scarce. Some wagon trains were lucky to have doctors, but this was an era where doctors would kill patients without doing anything wrong. Nobody really understood the stealth nature of microbial infection in the mid-1800s. Lack of this knowledge and lack of good sanitation on the trail meant doctors would transmit bacteria and germs around the wagon train, contaminating the patients they intended to heal. Add this to medical practices that we now know are useless (or worse, deadly), like the presence of mercury or opium in medicines, or “bleeding” a fever out of someone by opening a vein. While many doctors on the Oregon Trail were well-meaning and valuable, there were plenty of quacks, too.