Not necessarily. For example, some of the more recent SARS-CoV-2 variants spread more easily because the virus causes a more severe infection, which is transmissable for a longer period of time. It's also not a guarantee that the virus will evolve into something weaker. There are serious diseases like measles and smallpox that have circulated for thousands of years while remaining as deadly as they ever were. Sometimes, the disease not being as deadly after a while can be a result of natural selection selecting for more resistant individuals (after many deaths), not of natural selection selecting for less deadly pathogens.
> For example, some of the more recent SARS-CoV-2 variants spread more easily because the virus causes a more severe infection, which is transmissable for a longer period of time.
That's purely speculation at this point, while epidemiology shows us that viruses which are more fatal tend to be less transmissible and vice-versa.
It can be more complicated than that, because in some circumstances increased virulence might also provide an evolutionary advantage for the pathogen. This article from snopes.com has a nice summary of the competing views: https://www.snopes.com/news/2021/02/01/will-coronavirus-real...
"""The trade-off model recognises that pathogen virulence will not necessarily limit the ease by which a pathogen can transmit from one host to another. It might even enhance it. Without the assumed evolutionary cost to virulence, there is no reason to believe that disease severity will decrease over time. Instead, May and Anderson proposed that the optimal level of virulence for any given pathogen will be determined by a range of factors, such as the availability of susceptible hosts, and the length of time between infection and symptom onset.
There is little or no direct evidence that virulence decreases over time. While newly emerged pathogens, such as HIV and Mers, are often highly virulent, the converse is not true. There are plenty of ancient diseases, such as tuberculosis and gonorrhoea, that are probably just as virulent today as they ever were."""
