# Can a water lily kill you

## We lack a sense of exponential growth

Exponential growth can be heard - and the researchers show us a curve that points steeply upwards. But what does that mean? On March 3, there were 56 confirmed Covid-19 cases in Switzerland, on March 11, 652, and over 7,000 on the weekend. Nevertheless, most of them cannot understand the "exponential growth" emotionally.

That is normal. We just lack the sense of exponential growth. In contrast to linear growth. We are used to it because it happens all the time in front of our eyes. A baby gradually grows into an adult. Most things in our environment grow more or less continuously: a pet or a sapling that we plant.

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The situation is different with the growth of populations. Because not just a single organism grows here, but many individual ones. And feeling this kind of growth is extremely difficult.

Let's start with a single virus, it can also be bacteria, or water lilies in a pond. Each individual multiplies once a day. So on the second day there are two of them. Four on the third, eight on the fourth, and so on.

And now we assume that an entire water lily pond is completely covered on day 20. On how many days is half covered?

The solution: on the 19th day, half of the pond is covered. That means: on the last day the whole second half is added. That is exponential growth - and we have so little feeling for it. And that is why it is important that we do not listen to our feelings when fighting the disease, but to the mathematics.

Water lilies are an example of (nearly) exponential growth. If the whole pond is full on day 20, it was half covered with water lilies on day 19.

Philadelphia and St. Louis, and how the two American cities dealt with the Spanish flu in 1918, show what a stark difference it makes whether you take exponential growth seriously or not. Philadelphia took the disease easily. When the first flu cases occurred on September 18, the health department started a campaign against public coughing, spitting and sneezing. It was not until October 3rd that the city closed churches, schools, theaters and the like. Apparently too late: after a month, the city had almost 11,000 flu deaths.

Not so in St. Louis. They had learned from Philadelphia there. Quarantine was imposed just two days after the first flu cases appeared.

The impact on the number of deaths was striking. For every 100,000 people in Philadelphia, 257 people died per week at the peak of the epidemic, in St. Louis only 31.

So: the earlier you counteract exponential growth, the sooner you can flatten the curve - or in other words: the sooner you can save lives.

### The factist

The factist looks very carefully. He keeps an overview in the jungle of scientific study results. Shows what is related. And what just doesn't work. The factual is Beat Glogger, founder and editor-in-chief of higgs. Every Tuesday as a broadcast on Radio 1 and as a video on higgs.
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