Regarding eating fish and meat together. It seems from the sources that meat and fish should not be cooked together, nor eaten if it was cooked together because of danger to ones health. What is the principle behind this din?
What would be the Din if meat and fish were cooked separately but end up accidentally on the same plate (and touch each other), does one need to be strict and throw the food away? What happens if they are still very hot when placed on the same plate (and touch each other)? Is the din the same?
Answer:
Shulchan Aruch rules in two places that one may not mix meat and fish, even obligating washing hands between the two. However, some authorities rule that today the prohibition does not apply to the same degree, because we see that there is no danger involved in mixing meat and fish. Therefore, if the foods touch each other while in a cold state, one would be premitted to separate them, and eat them separately. However, if they touch one another while hot, so that each absorbs the ‘taste’ of the other, they should be discarded.
Sources: See Shulchan Aruch, Yoreh De’ah 116:2 and Orach Chaim 173:1-2; Magen Avraham (1) and Mishnah Berurah (3) write that the prohibition might be less severe today, but others, such as Shevus Yaakov 3:70) uphold the prohibition, and rely on the Magen Avraham only when there are other reasons for leniency. Therefore, for cold foods that come into contact, one may be lenient, but for hot foods, one should be stringent.
