Although Joe Biden tested negative for COVID-19 after potentially being exposed to the virus at the first presidential debate, official guidance suggests that he should be quarantining for 14 days. Party leaders are encouraging Joe Biden to keep his campaign going, despite his possible exposure to the virus which could be risking the health of other American citizens he comes into contact with. The virus' average incubation period is four to five days, and cases may not register on tests until after that time.
Therefore it is possible that Biden did catch the virus from Trump and it's just too soon to know. Biden's failure to quarantine following his potential exposure to coronavirus indicate that he may not be able to successfully virus-controlling policies such as isolating and distancing.