New Book: 'The Rules Of Contagion: Why Things Spread — And Why They Stop' : Goats and Soda - NPR

Adam Kucharski, associate professor at the London School of Hygiene and Tropical Medicine, is the author of The Rules of Contagion: Why Things Spread — And Why They Stop.

In studying it, we had two bits of information that are really hard to get at in disease outbreak: the reproduction number — how many others one person "infects" — and the lag time, 24 hours, after which people would stop spreading the game.

That reduces the reproduction number below one and leads to a smaller outbreak?

In the early stages of this pandemic, the reproduction number, or "R," was 2 to 3.

If you bring the "R" rate down below 1, it means one person infects fewer than one other person, and you can say the spread of the disease is under control and will eventually die out.

You can't think that when you get to an "R" number below 1, the problem is solved.

Some countries might think a predicted level of contagion is acceptable.

Other countries will think that same level of contagion is too high, and they need to put more measures in place for a longer time.

Contact tracing has been identified as a key element in stopping spread: finding all those in contact with a newly diagnosed case.

Can that work in other areas, like stopping the spread of gun violence.

Violent events can also spread through definable networks like gangs?

Other places, like Hong Kong and South Korea, are keeping the pandemic under control but with flare-ups.