It’s the “someone is in fact harmed” that is the tricky part in this case. If somebody who was previously asymptomatic got coronavirus a week after Bob walked past them, how do you propose to determine whether they got it from Bob? If you’re proposing a criminal penalty in the U.S., then you will need to prove this beyond a reasonable doubt. Probabilities of less than 99% won’t do, and there may be a few jurors who think 99.9999% still gives a reasonable doubt (the jury system has been described as “trial by people you would not normally consider your peers”).
It’s the “someone is in fact harmed” that is the tricky part in this case. If somebody who was previously asymptomatic got coronavirus a week after Bob walked past them, how do you propose to determine whether they got it from Bob? If you’re proposing a criminal penalty in the U.S., then you will need to prove this beyond a reasonable doubt. Probabilities of less than 99% won’t do, and there may be a few jurors who think 99.9999% still gives a reasonable doubt (the jury system has been described as “trial by people you would not normally consider your peers”).