US coronavirus cases have dropped 70% in the last six weeks. Experts chalk it up to more immunity and better behavior.

Andrew Noymer, an associate professor of public health at the University of California, Irvine, recently told Insider that the US may be underestimating the prevalence of asymptomatic infections. Indeed, a recent model suggests that just 13% to 18% of COVID-19 cases are symptomatic. Given that asymptomatic people are less likely to seek a test, this could indicate that official case counts are missing a lot of infections.