In my opinion, it is an unnecessary spectacle for the parents or the person. I think it has to do more with pride than avoiding illness. If the person was so allergic to wheat that they can't even eat a small chip, then they would be sick all the time, because there are more accidental pieces of wheat in all the food that they eat every day.

There certainly are many references to grains in the Bible, and with good reason. The Bible was compiled during a predominately agricultural time, and this would have been a reference that was easily understood by the people during that time period.

Though the Bible references grains, the grains consumed a couple thousand years ago bear little to no resemblance to the grains we consume (or don’t consume!) today.

In Jesus’ time, there were only three major types of wheat in existence: Einkorn, Emmer, and later Triticuм aestivum along with simple, non-hybrid varieties of other grains like barley, millet, and rye. These grains had (and still have) a higher protein content and lower anti-nutrient content than grains of today.

This is a stark contrast to the 25,000+ species that exist today, most of which we created in a lab to be disease resistant or produce high yields. In order to achieve these traits like disease and pest resistance, scientists had to enhance the part of the grains that naturally resists disease and predators: mainly, the glutens, lectins, and phytates- the most harmful parts of the grains to humans.

In addition, these hybridized strains are often allergen producing and usually sprayed with pesticides and chemicals. It is interesting to note that some people who are allergic to modern strains of wheat show little to no reaction to (properly prepared) Einkorn wheat in small amounts.

So, the grains of Jesus’ time weren’t genetically similar to the grains of today, and had lower concentrations of the harmful com
ponents. In addition, they were prepared much differently:

In the Eastern Rite, there's no problem, since people receive under both species.

But I'd really love to know what gluten free host the SSPX would be using.  For validity, wheat must be used, and I know of no gluten-free varieties of wheat.

However, in the example the OP gave, the priests is giving a special communion to the person. I have seen it done, the priests after he is done with everyone, goes up to the tabernacle and brings back the special host just for that one person. It is quite a spectacle on Sundays.