Isn't this a trend in biology? Where less virulent strains of viruses out-perform more severe ones because they don't kill their hosts? Ebola is its own worst e

Isn't this a trend in biology? Where less virulent strains of viruses out-perform more severe ones because they don't kill their hosts? Ebola is its own worst enemy because it kills its hosts, and kills them relatively quickly. I seem to recall reading that influenza or typhoid had become less dangerous in the last 100 years or so and they were attributing it to this, but I'm coming up blank on the google searches.