If according to modern science and medicine the mixture of fish and meat is not dangerous, why do we still follow it? Is it crticial that we avoid this combination, if the reason it was given was for health?
Our sages of previous generations were obviously not privy to all of the scientific information we possess today (imagine, what did they do all day without TV or internet?). Rather, their expertise is an area of knowledge far more profound: understanding G-d’s revealed Will in creation.In certain areas, our sages record a tradition to be careful about certain things for what are apparently health reasons, such as your example: not eating fish and meat together. It is often not clear whether they received this tradition from some Divine inspiration or merely from the medical presumptions of their time. These traditions are recorded because of the mitzvah to carefully guard one’s health (thus an observant Jew must always follow the best medical advice, not only when illnesses arise, G-d forbid, but even in our day to day diets and routines.)
More recent authorities take different positions on the ruling regarding fish and meat. Some authorities do indeed hold that this law does not apply anymore because it seems that our physical natures have changed and this combination no longer poses a danger.
Most authorities, however, hold that the law still applies. How can they take such a position in light of modern science?
For two reasons.
One, it is possible that fish and meat eaten together does pose a health risk, but we, at present, are not aware of what that risk is. Many ailments exist today for which there is no clear known cause. Even with diseases which have certain known risk factors, why do some people exposed to those factors contract the condition and many others do not? It is possible that this combination of foods is an additional risk factor in certain diseases. Also, it is possible that this combination of foods truly adversely affects only a very small percentage of eaters (let’s say, 1 in 10,000). Such a small percentage would be ignored as statistically insignificant in a scientific study, but according to Jewish Law, one life in 10,000 or even a million or more is still infinitely significant.
Two, the Vilna Gaon stressed that every item that our sages codified as prohibited has many reasons for its prohibition. The sages may only have listed an overt reason to prohibit something, but there are mystical reasons as well to refrain, which they did not see as fit to publicize. Even when the overt reasons become obsolete, the mystical reasons still apply.