Where do these people come from -- everywhere! Riders will come from thousands of miles away and even from other countries. What do they do when they get there? One word: "Party!" Think of the event as a multi-day frat party for seniors. As the Harley-Davidson company can tell you, sales are down due to younger generations of potential riders not taking up the "open road" on a bike. Perhaps there will be more canes in the bars. Useful in a fight!
Here's a description of the event from USA Today, No masks required as 250,000 expected at 10-day Sturgis Motorcycle Rally.
In unscientific terms, riders will travel across the United States, with those who already have COVID-19 stopping in bars, restaurants and hotels. Then they will spend multiple days in crowded indoor and outdoor spaces with other thousands of people in intimate contact. After spreading the virus around, they will get back on their bikes for the ride home. Those infected will then repeat the spread on their trip across multiple states.
Watch for a significant spike in hospitalizations in South Dakota in the coming weeks.