Excerpts.

At the University of Illinois at Urbana-Champaign, more than 40,000 students take tests twice a week for the coronavirus. They cannot enter campus buildings unless an app vouches that their test has come back negative. Everyone has to wear masks.

This is one of the most comprehensive plans by a major college to keep the virus under control. University scientists developed a quick, inexpensive saliva test. Other researchers put together a detailed computer model that suggested these measures would work, and that in-person instruction could go forward this fall.

But the predictive model included an oversight: It assumed that all of the students would do all of the things that they were told to.

Enough students continued to go to parties even after testing positive, showing how even the best thought-out plans to keep college education moving can fail when humans do not heed common sense or the commands from public health officials....

What the scientists had not taken into account was that some students would continue partying after they received a positive test result. “It was willful noncompliance by a small group of people,” Dr. Goldenfeld said.

Those were the key ingredients for a few people infecting many others. “If you know you are positive,” Dr. Elbanna said, “and you go to a party, that’s not just a bad act. That’s very, very dangerous.” ...

Comprehensive testing of everyone on campus and prompt contact tracing showed the trouble spots — some fraternities and sororities, as well as some off-campus housing, that were throwing parties — as well as where the containment plans were working. There were few signs of the virus spreading in classrooms or from students to the people in the surrounding towns of Champaign and Urbana....

Dr. Goldenfeld said the main purpose of the model was not to make precise predictions, but to help administrators make informed choices on what precautions made sense.

For example, the model showed that once-a-week screening, as university administrators originally planned, was too little, too slow. Students who were infected soon after a test cleared them would be infectious for days before the next test. The university increased the mandate to two tests a week, although now that schedule is only for undergraduate students.

Since the university clamped down last week, the number of new cases has dropped again, and the hope is that all students will now take the protocols more seriously....