The only incident of sickness coming from communion I'm aware of is when King Pap poisoned the lamb (Eucharist before consecration) of Catholicos Sahak of Armenia.
I think you got the story a little mixed up. It was Pap's mother, the wicked queen Parandzem, who bribed a corrupt priest to kill someone through poisoning the Eucharist. Pap did later poison a catholicos, but it was St. Nerses, who was the father of St. Sahak, and it was with a cup of ordinary wine at a banquet.
I'm kind of surprised you know the story at all. It's one of those juicy, gossipy stories you find in history, but it's so obscure that almost no one hears of it.
The way I heard the story, Queen Parandzem didn't start out wicked. She was more of a victim at the beginning. She was the very beautiful wife of a nobleman, and the nobleman's cousin wanted her. The cousin told lies about Parandzem's husband to the king, so the king would kill him and the cousin could take her. After the king killed Parandzem's husband, however, he saw Parandzem and decided to take her for himself.
Parandzem hated the king and did everything she could to make him miserable after he married her. So he took a second wife, a Greek princess named Olympia. (This was in the 300's, when Christianity was still new in Armenia. It took a couple of generations for polygamy to fall out of use.) Parandzem hated Olympia and bribed a corrupt priest to murder Olympia by poisoning the Eucharist. That was the horrible deed that made Queen Parandzem go down in history as one of the most wicked queens ever.
Later, the Persians invaded Armenia and Queen Parandzem was brutally raped and killed by Persian soldiers. Her son, Pap, was out of the country and safe at the time.
Later, when Pap became king he was himself a very wicked ruler. The Catholicos St. Nerses the Great reprimanded Pap a number of times for his behavior, which made Pap resent St. Nerses. So he invited the saint to a banquet and gave him a poisoned cup of wine. When St. Nerses drank the wine, he realized he had been poisoned and before dying he praised God for allowing him to be a martyr. He also asked God to forgive Pap.
This was the event that resulted in the Armenian Church being an independent Church from Caesarea. When Pap picked a replacement for St. Nerses and sent him to Caesarea to be consecrated, St. Basil, who was the patriarch there, refused.
Anyway, I always wondered how Olympia could have been poisoned through the Eucharist. Perhaps the corrupt priest did not properly consecrate it, so it wasn't really the Body and Blood of Christ? It's a horrible story, but I think it was written down not too long after it happened by a historian (Moses Khorenatsi maybe?) so I don't think scholars doubt that it happened. It's just that I'm wondering how
it could have happened.