Hi. My name is Eugenio and I’m a Professor at the *Departamento de Didáctica de las Ciencias Experimentales, Sociales y Matemáticas* at the *Facultad de Educación* of the *Universidad Complutense de Madrid (UCM)* and a member of the *Instituto** de Matemática Interdisciplinar (IMI) *of the *UCM*.

I have a 14-year-old son. In the beginning of the pandemic, a confinement was ordered in Spain. It is not easy to make a kid understand that we shouldn't meet our friends and relatives for some time and that we should all stay at home in the first stage. So, I developed a simplified explanation of virus propagation for kids, firstly in *Scratch* and later in *Maple*, the latter using an implementation of turtle geometry that we developed long ago and has a much better graphic resolution (E. Roanes-Lozano and E. Roanes-Macías. An Implementation of “Turtle Graphics” in Maple V. *MapleTech*. Special Issue, 1994, 82-85). A video (in Spanish) of the *Scratch* version is available from the* Instituto de Matemática Interdisciplinar (IMI)* web page: https://www.ucm.es/imi/other-activities

**Introduction**

Surely you are uncomfortable being locked up at home, so I will try to justify that, although we are all looking forward going out, it is good not to meet your friends and family with whom you do not live.

I firstly need to mention a fractal is. A fractal is a geometric object whose structure is repeated at any scale. An example in nature is Romanesco broccoli, that you perhaps have eaten (you can search for images on the Internet). You can find a simple fractal in the following image (drawn with *Maple*):

Notice that each branch is divided into two branches, always forming the same angle and decreasing in size in the same proportion.

We can say that the tree in the previous image is of “depth 7” because there are 7 levels of branches.

It is quite easy to create this kind of drawing with the so called “turtle geometry” (with a recursive procedure, that is, a procedure that calls itself). Perhaps you have used *Scratch* programming language at school (or *Logo*, if you are older), which graphics are based in turtle geometry.

All drawings along these pages have been created with *Maple*. We can easily reform the code that generated the previous tree so that it has three, four, five,… branches at each level, instead of two.

But let’s begin with a tale that explains in a much simplified way how the spread of a disease works.

- o O o -

Let's suppose that a cat returns sick to Catland suffering from a very contagious disease and he meets his friends and family, since he has missed them so much.

We do not know very well how many cats each sick cat infects in average (before the order to STAY AT HOME arrives, as cats in Catland are very obedient and obey right away). Therefore, we’ll analyze different scenarios:

- Each sick cat infects two other cats.
- Each sick cat infects three other cats.
- Each sick cat infects five other cats

**1. Each Sick Cat Infects Two Cats**

In all the figures that follow, the cat initially sick is in the center of the image. The infected cats are represented by a red square.

**·** Before everyone gets confined at home, it only takes time for that first sick cat to see his friends, but then confinement is ordered (depth 1)

As you can see, with the cat meeting his friends and family, we already have 3 sick cats.

**·** Before all cats confine themselves at home, the first cat meets his friends, and these in turn have time to meet their friends (depth 2)

In this case, the number of sick cats is 7.

**·** Before every cat is confined at home, there is time for the initially sick cat to meet his friends, for these to meet their friends, and for the latter (friends of the friends of the first sick cat) to meet their friends (depth 3).

There are already 15 sick cats...

**·** Depth 4: 31 sick cats.

**·** Depth 5: 63 sick cats.

Next we’ll see what would happen if each sick cat infected three cats, instead of two.

**2. Every Sick Cat Infects Three Cats**

**·** Now we speed up, as you’ve got the idea.

The first sick cat has infected three friends or family before confining himself at home. So there are 4 infected cats.

**·** If each of the recently infected cats in the previous figure have in turn contact with their friends and family, we move on to the following situation, with 13 sick cats:

**·** And if each of these 13 infected cats lives a normal life, the disease spreads even more, and we already have 40!

**·** At the next step we have 121 sick cats:

**·** And, if they keep seeing friends and family, there will be 364 sick cats (the image reminds of what is called a Sierpinski triangle):

**4. Every Sick Cat Infects Five Cats**

**·** In this case already at depth 2 we already have 31 sick cats.

**5. Conclusion**

This is an example of exponential growth. And the higher the number of cats infected by each sick cat, the worse the situation is.

Therefore, avoiding meeting friends and relatives that do not live with you is hard, but good for stopping the infection. So, it is hard, but I stay at home at the first stage too!