It happens quite often, depending on your definition of "kneecap".
SARS-1 mutated to be less lethal and no longer a concern. Ebola in Africa did similarly, though "less" lethal is not "not" lethal so I'm not sure on your definition of kneecapping, it was/is still quite nasty. The plague kneecapped itself in the other direction, becoming too lethal too quickly, and so was unable to spread well since when contagious symptoms showed, everyone buggered off for a couple hours for the person to die.
It happens quite often, depending on your definition of "kneecap".
SARS-1 mutated to be less lethal and no longer a concern. Ebola in Africa did similarly, though "less" lethal is not "not" lethal so I'm not sure on your definition of kneecapping, it was/is still quite nasty. The plague kneecapped itself in the other direction, becoming too lethal too quickly, and so was unable to spread well since when contagious symptoms showed, everyone buggered off for a couple hours for the person to die.