The Knesset Hagedola (Choshen Mishpat 333:17) says that even the Rosh, who says that a worker who is sick does not deserve payment, agrees that if a worker cannot come to work, not because he is sick but in order to avoid an epidemic, he does get paid. He posits that this is not considered to fall under the category of one who is unable to work because of illness. Rav Moshe Taveretzky (Kol Hatorah compendium, vol. 54) understood within the Knesset Hagedola’s opinion that if a teacher could have pushed himself and found the strength to teach and he did not do so, he would not receive his salary. However, this does not seem to be the case, as the Knesset Hagedola does not indicate this and it is unlikely that we should make such distinctions, which are hard to discern from case to case.
Mahari Bruna (134) and the Tzitz Eliezer (II, 26) hold that when a worker is sick but is able to work with a lower level of efficiency (perhaps even with reduced hours) he receives full pay. Their proof is from the gemara regarding an eved ivri, which says that although an eved who is sick for more than three years has to make up the time, if he was able to work at light jobs, he does not have to make up the time. The Tzitz Eliezer assumes that the same is true of a regular worker.
However, logically there is room to distinguish between the gemara’s case and that of a regular worker. An eved does not come to an agreement in advance with his owner as to which tasks he will be responsible for. Rather, he is obligated to do everything that he can that the owner wants. Therefore, whatever work he is capable of doing is considered a fulfillment of his servitude. In contrast, a standard worker is hired for a specific job. If he is unable to do this job according to the minimum specifications, then he is not fulfilling his obligations and may not deserve full salary. This is the implication of the Ritva (Kiddushin 17a).
Let us now summarize that which we have learned. An eved ivri does not have to make up for time he misses due to illness unless he misses more than half of his stint as an eved. There is a machloket Rishonim regarding if a regular worker has the same parameters in this regard as an eved or whether he normally is not paid for the work he misses due to illness. Exceptions to the rule would be in places where there is an accepted practice to pay for sick days and in a case where it is evident from the situation that the employer relinquishes his rights to compensation for the work missed. It also appears that if illness makes a worker perform at a lower than expected level of efficiency, he is also paid in full.