All parties concerned accept that Sakyamuni Buddha, or Gautama Buddha, formulated a number of rules for his monks, and that a fair number of rules were added over time, either by Buddha himself or by vínaya masters in later ages.
With 'sangha' we mean the fully ordained Buddhist monks and nuns according tot the orthodox rules contained in the above mentioned vínaya and it's condensed set of rules, the pātimokkha or prātimokhsha (in Pali language resp Sanskrit).
A series of awkward questions
The 42-year old Chinese actress Chen Xiaoxu, who gained fame as one of the main characters in a Daoist inspired movie based on the Classic Dream of the Red Chamber, entered monk/nunhood on February 26, 2007. She died of breast cancer on the 13th of May that same year.
Aspirant monks and nuns cannot enter full monkhood when ill or otherwise incapacitated.
Not because they're evil, but because one should in terms of health care not be a burden to the laity.
Just before the candidate enters the sīma, the consecrated area in which formal ordination takes place a series of rather awkward questions is asked. These questions are standardised and formalised, and rather, but not too, ancient: Are you a human? Are you indebted? Are you a runaway prisoner? Do your parents agree? Are you married? Do you have boils? (and other questions regarding one's physical health and whether the candidate has the genital characteristics of a hermaphrodite).
Chen Xiaoxu got away with it because 'Do you have a cancer?' is not on the list, nor is 'Do you have tuberculosis?', 'Are you hiv-positive', 'Do you suffer from Alzheimer?' and other such ailments that today are more common than boils, i.e. pestilence.
This series of questions that have to be asked by nuns, because answering them in front of a chapter of monks is too embarrassing(*), clearly show an 'add-on' structure. Every time a new problem arose, a question regarding this problem was added to the standard list.
When at a certain point in time the bhikkhuni (nun's) Order reached extinction, this series of questions was no longer updated. Now that bhikkhuni/bhikshuni ordination is well under steam again, this list has not yet come under today's vínaya master's scrutiny. It should.
(*) This remark was justly made by bhikkhuni Dhammananda, one of contributors at the above conference.