The virus "cares" if it reproduces. There is often tension between the various levels of spreading mechanisms: for example airbourne spread diseases making you cough vs. the cough making the host feel poorly and not interacting with people or the cough killing the host really preventing further spread. There are plenty of optimum points between fast intense disease and asymptomatic disease.

Short term intense disease courses tend to only work for a short period of evolution for new infection mechanisms, the intensity makes them sensitive to any increased immunity which ends up halting the spread for more mild versions. Infectious diseases tend to lower in intensity over the long term.