Great question. Scientists who study disease—epidemiologists—look at two different measurements to determine which is worst.

One measure is how fast it spreads. In other words, if one person has it, how many other people will they give it to? This is often called R-naught. You can just say, “How contagious is it?”

For example, measles spreads really easily. One person will spread it to 12-18 people on average. COVID-19 isn’t all that contagious. One infected person will spread it to about 2.5 people on average if no one is being safe. This number can change. For example, if people stay home, wear masks in public, and wash their hands, this number can go below 1, and the number of cases will go down. Or the contagion number can go higher if people are out breathing and coughing in crowds. A vaccine could bring the number way down, but we don’t have one yet.

Any time the contagion number goes above 1, the number of cases will grow faster and faster. That’s called exponential growth. Think about it this way, if you tell 2 people a secret and each of them tells two people, and so on, it’s not a secret anymore.

The other number scientists look at is how bad the disease is. Case fatality rate (CFR) is the percentage of people who get the disease that die. It doesn’t include people who are disabled for a long time. Ebola has a CFR of 65%-100%. Fortunately, Ebola is not very contagious. There are some nasty diseases with a fatality rate close to 100%. Rabies has a very high CFR. Fortunately, it is not easy to get, especially now that people vaccinate their pets against rabies.

In the winter of 2018-2019, the CFR for influenza (flu) was about 0.1 %. Right now (fall of 2020), the CFR for COVID 19 is about 3%. This number can go down as health care workers learn how to treat the disease better. It can go up if there are not enough hospital beds or staff to help people. A cure would bring this number way down, but we don’t have one yet.

There isn’t really a good way to combine the two numbers. Remember that measles spreads fast. But the CFR for measles is only about 0.15%, or 1-2 people per 1000 who get it. We also have a vaccine for measles, so you don’t usually hear about measles unless it hits a community where a lot of people don’t get the vaccine. Ebola has a high CFR, but a contagion rate of about 1.5, which is pretty low.

Which do you think is more important, developing cures, or developing vaccines?