“If the numbers were staying stable or they were going down, I’d have lots of confidence in the plan,” Halkitis said of the N.B.A.’s restart. “The numbers going up mean you have a difference circumstance now, which increases the probability of transmission and makes the plan — which is excellent but not foolproof — more susceptible to infiltration by the virus. “I keep using the flood and the dam example: A dam holds water, but if there’s a lot of pressure on the dam, like lots of infections, it’s more likely to crack. And that’s the problem here. They have to keep an eye on what’s going on.”