They can go from "harmless" to "deadly" via one mutation or several mutations.
If you look at the Swine Flu Virus as an example, you will understand it better.
It was discovered by the US CDC scientists in 2005. Was pretty harmless then and infected few people.
Between 2005 to 2009, only 12 people got infected and all got better. But a mutation occurred in 2009, which caused it to spread like wildfire and killed over 500,000 that year.
This is the nature of viruses: they can go from good to bad or bad to good via a simple mutation.
As a side note, we are less than 2% mutation from the Ape.
Yet, that small mutation has enabled us to rule the world, whilst the Apes are endangered from extinction.
