In the past two decades, coronaviruses have produced three deadly global epidemics: SARS, MERS and now Covid-19. If you’re wondering what it is about this family of viruses that makes it produce such deadly pathogens — and what about them might hinder their spread — here are a few things you should know.

1. They have a high substitution rate.

Coronaviruses have, like many RNA viruses, a high substitution rate (though the rate is lower than some other viruses that have caused pandemics, like HIV or hepatitis C). This higher substitution rate means that coronaviruses can rapidly exploit situations in which they come into contact with new hosts (i.e., humans).

2. They mutate rapidly.

One reason coronaviruses mutate so rapidly is that they possess the longest genomes of all known RNA viruses. With more sections in their genome, there are more potential errors when the virus copies itself, which increases the production of new strains (which may explain the recent finding that there are likely to be multiple Covid-19 strains).

3. They are highly susceptible to recombination.

One of the ways viral strains emerge is through recombination, which is when multiple viruses interact in the same organism (e.g., humans) during replication. Research predating the emergence of Covid-19 has shown that, of all the many coronavirus strains that are out there, a human coronavirus known as HCoV-HKU1 is among those most highly susceptible to recombination. It is also one of the coronaviruses most closely related to Covid-19, along with SARS and MERS: all are members of the subset of betacoronaviruses.