Mark and Lucas McCain come across a covered wagon just sitting off by the road. Inside, they find a young couple who are burning up with fever. Lucas takes them back to the ranch and Mark fetches the doctor who confirms that the couple are suffering from highly contagious yellow fever. As word spreads through North Fork, panic sets in stirred up by Brett Conway who Lucas had recently refused to hire. A mob soon descends on Lucas' ranch intent on burning it to the ground.
—garykmcd