There are plenty of ways it could be a sneeze, a handshake, to name a few.

Then there's our habits washing our hands, not touching our face.

Even the climate may play a role in determining how far and how fast and infection spreads.

Scientists have summed up how far an infection can spread with the number it's called Arnold.

A number greater than one indicates it'll grow less than one indicates the outbreak will die out.

Scientists determine the number with a formula that brings together all the environmental and biological factors.

This is how it looks.

In reality.

An infection is introduced to human from an animal.

Let's say the bigger they are not number, the more people of virus can infect.

So an R nought of two means that patient zero will on average, infect to other people who in turn can infect another two people and so on.

On infection, like measles, sits higher of the spectrum with an R nought of between 12 and 18.

At the lower end is something like Ebola, with an arnold number of 1.5 to 2.5.

And while the flu varies from year to year, one study reports.

It averages to 1.2.

But as various factors change, so too does the Our note takes ours.

When it first emerged, SAR spread rapidly with a high Arnold, but people were only contagious.

One symptoms started showing.

Once people were informed about the symptoms, they started coming to the hospital Earlier, people could check themselves into hospital before infecting other people, pushing the Arno number below one, and that is how a virus can spread.