[throwaway2245]

The most-efficiently-replicating strain is likely to dominate in the long-term. As such, there is essentially no evolutionary advantage to killing or harming humans outside of the goal of replicating more efficiently. (There is also no evolutionary disadvantage).

So, there are two suggestions within viral genetics:

1. Mutations which are kinder to the immune system might be able to spread relatively easier. As noted in the other comments, this is likely dependent on human behaviour and ritual around illness and death.

2. Significant mutations which spread more efficiently are perhaps more likely to knock-out genes causing harm (lost to 'evolutionary cost') than to enhance those genes.

Neither of these are completely guaranteed, but they are generally seen as likely.