My god, you can be thoughtless. Here's one way to be more thoughtful: when a client emails you to tell you that they have tested positive for Covid, you can say something like, "I hope you feel ok" or "take good care of yourself" or "I am thinking of you". Feel free to craft your own responses, but these are useful templates for how a caring individual might respond. An uncaring individual might say something like, "thanks for letting me know". Oh wait, does that sound familiar? It does? Yes, that's because that is all you could muster. "Thanks for letting me know". As if I was telling you that you had a flat tyre. Cretin.