Some time ago, I had a discussion on the Duolingo forums about the relativity of verb tenses in Esperanto. It seemed in that discussion that I had always been slightly mistaken about my understanding of the rule. I thought the verb tense in a subordinate clause takes as its point of reference the tense established by the main verb, so that if you want to say that you knew at past time X that a person was sick at that same time X, the subordinate clause would take the present tense, because it would be present relative to the time of the main verb. However, it was pointed out to me that the tenses are wholly independent, such that if the subordinate clause is also at time X, i.e. also in the past, it would therefore just be past tense. (The difference with English would then be that in English, something continuing into the past would still be given in past tense if the main verb is in past tense, which would not be the case in Esperanto.)
So I would have originally said, "Mi ne sciis, ke vi estas malsana." According to the other person, it should be "Mi ne sciis, ke vi estis malsana", unless the person is still sick, and then it could be "Mi ne sciis, ke vi estas malsana".
Is this indeed correct? I suppose it feels strange to me in that last case that you would be talking about knowing in the past about the illness in the present, i.e. the future relative to when you were doing the knowing, as it were. Also, it could lead to awkward ambiguities, where in "Mi ne sciis, ke vi estis malsana", it is not clear if the sickness was at the same time of the knowing, or further in the past still (before time X). The person suggested "estis estinta malsana" or even "estis malsaninta" as possible ways to specify that it's further in the past, but those both feel very clunky, to me.
PMEG (relevant page) seems to support what the other person was saying, but I think it leaves a little bit of room for uncertainty, and I wanted to check up on it further with the expertise of other users of this site, just to be on the safe side.