Viruses, like COVID-19, evolve rapidly. Each time the virus replicates, mutations can occur in its genome. Most of these mutations have no effect, or are even damaging to the virus. However, occasionally a mutation will arise that is advantageous for the virus. These mutations may allow the virus to grow faster, spread better or evade our immune system. The longer a virus continues to circulate, the greater the chance of these mutations occurring and the virus evolving into a new strain that behaves differently.
However, causing more severe disease isn’t necessarily advantageous to the virus. One of the reasons the COVID-19 virus is so difficult to contain is that it spreads very well before people become sick. If the virus spreads while causing more severe symptoms, people with COVID-19 would likely stay home instead of going out (reducing transmission) and would seek medical attention (enabling more effective testing and contact tracing). Sometimes mild viruses are the most difficult to eliminate.
Because the COVID-19 pandemic has been so difficult to contain, we have all had to take precautions, such as social distancing, washing hands and wearing masks. Each of these individual precautions limits the amount of virus that can spread from one person to another. These actions could put pressure on the virus to evolve, possibly resulting in mutated strains that are more transmissible and more difficult to control (though not necessarily deadlier). However, by combining all of these precautions, together with strong public health infrastructure (such as test and trace), we can still effectively stop the transmission of COVID-19, even if it evolves into a more virulent strain.