As a Harvard University official told MSNBC’s Lawrence O’Donnell back in March, “The dorms are cruise ships.” Even if sophisticated testing uncovers a case of COVID-19 within a few days of a student contracting it, that student is likely to have come into contact with dozens of others in the intervening days.

Therefore, many colleges will likely, within weeks of reopening, place a quickly expanding set of students under lockdown. And if these measures fail, the colleges will close on short notice. At that point, thousands of students—many of them infected with COVID-19—will board trains and planes to go home, spreading the virus to their families.

If colleges reopen, kids from parts of the country with high case counts will, inevitably, travel to parts of the country with low case counts—and bring their home-state problems with them. This is why the biggest threat posed by reopening colleges is not to students, faculty, or staff, but to the surrounding community.