How one case becomes thousands: the reproduction number

0:00How can one sick person lead to thousands of cases?

0:04Suppose each sick person passes the infection on to 2 other people. That is the first round of new cases.

0:13The average number of people that one sick person infects is called the reproduction number, or R.

0:20In this example, R is 2.

0:23Each round doubles the one before: 4 new cases, then 8, then 16.

0:31If it keeps doubling, the tenth round alone has 1,024 new cases.

0:37Add up all the rounds, and more than 2,000 people have been infected.

0:43So a few cases are worth taking seriously: they can grow into an epidemic, a disease spreading to many people in a short time.

0:52Whenever R is above 1, each round is bigger than the last, so the number of cases grows.

0:59Some diseases have a much bigger R. Measles is one of the most contagious diseases, which means it spreads very easily.

1:09Where nobody is immune, or protected against it, one person with measles infects about 12 to 18 others.

1:18Some ways to lower R are handwashing, vaccines and staying home when sick.

1:25Suppose they bring R down to 0.5: only half of the sick people pass it on, to one person each.

1:33Now each round is half the size of the last: 8, then 4, then 2, then 1.

1:41When R is below 1, each round is smaller than the last, so the cases fade out.

1:47So R above 1 means cases grow, and R below 1 means they fade out.