The story is that a discarded eucharistic bread piece (host) was left in a church. When the priest found it, he put it away. The next day, in place of the host there was a much larger piece of bloody substance. Years later, an Argentinian Cardinal, now the Pope, investigated this and ordered that the still fresh flesh be scientifically tested. A sample was sent to a famous medical examiner in New York, Dr. Frederic Zugiba, who proved the tissue was human cardiac muscle from a left ventricle.
There are numerous Catholic sources on the internet that tell that story, almost word for word. But there is woefully little respectable, secular coverage of the events depicted. In searching for additional information I looked up the doctor. His name is actually spelled Dr. Frederick Zugibe and he is a recently deceased, famous medical examiner for Rockland County, New York. But nowhere in his publications, biographies, or other online information officially attributed to or about him is any mention of this miracle story from Argentina. There’s plenty about his Shroud of Turin investigations and crucifixion studies, but I found not a single reliable source of documentation that corroborates his involvement with bread turning into human cardiac tissue, just a blog post with no citations.
I place a very low confidence in the accuracy of that miracle story. The story itself shares many characteristics in common with internet chain letter glurge. Various sources of the story are all self referential and nearly verbatim copies of one another. I could find no secular reporting on the story that doesn’t just credulously repeat the internet story and very little of that. There is no way to guarantee that no one could have tampered with the bread/flesh at any point in the years after its discovery but before samples were allegedly tested. But most importantly, the story makes fantastic claims that would be hard to believe even if the National Academy of Sciences had throughly investigated and endorsed all the particulars, making this a sensational, international news story.
No, it is far more likely that the story is fabricated in part or in whole and that pious fraud has played a role in aspects of the story’s development and distribution. Which is how I'll treat this story and any others like it, until such a time that the preponderance of solid scientific evidence serves as irrefutable, if still extraordinary proof.