RO number

The R0 (R-zero or R-nought) is an important factor. It is a number starting from 0 and is defined for every infectious disease.

The R0 number denotes how many persons an infected person can infect.

A value between 0 to 1 means that the virus will not spread because it cannot affect many people.

When does an infection become a pandemic kind? It is when R0 reaches 4 or above.

Following are the R0 numbers of the common infectious diseases (Source Basic reproduction number - Wikipedia):

COVID-19 is at the moment on number 4 position. Now we know why SARS fizzled out. Its R0 factor is near 1 and many times less than 1.

There is also another factor. This is how many deaths occur per 1000 infections.

A parasite can survive if the host survives. MERS and EBOLA had a very high “killing rates”. That is why they did not spread wide.

COVID-19 spreads (like influenza) but does not kill that many.