The Mishna at the beginning of the perek (73b) enumerated five specific activities that are forbidden on Yom Kippur in order to fulfill inuy – the commandment to reach a sense of suffering or oppression. Therefore, most of these activities are forbidden only if they are done for pleasure. Thus, the baraita on our daf teaches that someone who is dirty is allowed to wash himself and someone who has sores on his body can anoint them with oil. Rabban Shimon ben Gamliel is quoted as teaching that a woman who needs to feed her children can wash one hand so that she can give them food.
They said about Shammai the Elder [HaZaken] that he did not want to feed his children with one hand, to avoid having to wash it. This prevented the children from eating during all of Yom Kippur. Due to concerns about the health and the suffering of his children, they decreed that he must feed them with two hands, forcing him to wash both hands.
Most of the commentaries explain that Shammai HaZaken (the Elder) was reluctant to rely on the “leniency” and wash his hand. The Sages ruled that he should therefore wash both his hands, because they wanted to emphasize that, in this case, there was no prohibition at all. The Ritva points out that there are several similar cases in the Talmud, where the Sages went beyond the letter of the law in order to emphasize the correct ruling. Rabbenu Yehonatan understands this case differently. He argues that Shammai HaZaken was concerned lest he touch the food with his unwashed hand, so he refrained from feeding his children entirely. The Sages reacted to this by permitting him to wash both hands.
What was the great concern about touching food?
The Gemara quotes Abaye as explaining that the Sages were afraid of shivta. Rashi explains that shivta is a ru’ah ra’ah – an evil spirit. According to the responsa literature from the period of the Ge’onim, shivta was a disease that affected mainly babies and younger children. From the descriptions that appear in the Gemara it seems likely that it is some type of contagious infection that can be carried by dirty hands.