>> yet it hasn’t weakened in all these decades <<
Are you referring to Ebola? Indeed, it does appear to have weakened. The death rate used to be around 90%. The death rate in the current outbreak is reported to be about 65%.
The sequence of the current strain exhibits some drift from the 1976 version. Lower mortality but easier transmission may be the result.
Something that’s got 90+% mortality but isn’t easy to transmit is a very different bug from something that’s got 30+% mortality but is much more easily transmissible.
The former burns itself out. The latter is a pandemic.
I found that out after I posted.