The Catholic Church approved the beatification of the late Pope John Paul II after a Catholic nun, Marie Simon-Pierre, reported being cured of Parkinson's disease after praying to John Paul II. Wikipedia reports that the validity of the beatification has been criticised on a number of grounds, including a suggestion that the unusual rapidity with which it proceeded might be intended to deflect criticism of John Paul's failure to respond effectively to the scandal of sexual abuse.
To be certain a miracle cure occurred, we would have to be certain Sister Marie Simon-Pierre ever really suffered from Parkinson's disease, but this has yet to be proven. The disease usually affects people much older than Simon-Pierre and can not be demonstrated with certainty other than by an autopsy. There is no lab test that will clearly identify the disease, but brain scans are sometimes used to rule out other disorders that could give rise to similar symptoms. The progress of the illness over time may reveal it is not Parkinson's disease, and some authorities recommend that the diagnosis be periodically reviewed. People diagnosed with Parkinson's may be given levodopa and resulting relief of motor impairment tends to confirm diagnosis. It would seem that either Simon-Pierre was not given this treatment, or that it was ineffective, since she says, "From April 2, 2005, I began to worsen week by week, I grew worse day by day." This does not necessarily prove that she did not have Parkinson's disease, but is one ground for doubt.
Diagnosis of Parkinson's disease can at best be established from the presence of a number of indicators, two of which are response to levodopa for at least five years and clinical course of at least ten years. Sister Simon-Pierre reports a period of around four years from initial diagnosis to cure, but does not mention levodopa treatment, so this does not tell us whether this test was considered. Again, since she only had symptoms for four years from the time of diagnosis, the second text fails. It is possible, particularly given the early onset (Most individuals with Parkinson's disease are diagnosed when they are 60 years old or older, but early-onset Parkinson's disease also occurs), that Sister Simon-Pierre suffered from another neurological disease which has similar symptoms as Parkinson's but which can be cured.
Sister Simon-Pierre insisted that her cure was complete and instantaneous, but has never publicly explained who first diagnosed her with Parkinson's disease and how that diagnosis was confirmed. After the miracle, we of course have investigations that show her free of the disease, although there are unconfirmed reports that she subsequently had a relapse. What is really important is not whether she is free of Parkinson's disease now, but whether she ever suffered from the disease in the first place.