Prologue Cancer results from a prologue which grows far beyond its intended size. This is often the result when a writer doesn’t understand the purpose of the prologue, and puts in carcinogenic content, bloating the prologue until it chokes off the first chapter, which the audience will never read.
Preventing prologue cancer is easy if you understand the purpose and scope of a proper prologue. From literarydevices.net, we get:
The prologue, Greek prologos (meaning: before word), is an opening of a story that establishes the setting and gives background details.
Generally speaking, the main function of a prologue tells some earlier story and connects it to the main story. Similarly, it is serves as a means to introduce characters of a story and throws light on their roles. In its modern sense, a prologue acts as a separate entity and is not considered part of the current story that a writer ventures to tell.
From this definition, it is easy to see that the prologue is supplemental material. If the prologue can be removed without affecting the rest of the story, then it is a true prologue. But if the prologue is used to provide the opening scene, or introduce the main characters at the beginning of the action, then it is bloated beyond its intended purpose, and needs to be cut out like the cancer it is. Or, sometimes, rename it Chapter One.
There are many readers who skip over the introductory pages of a story, including the copyright page, title page, the map at the beginning, the dedication, special thanks, the note from the author, and the prologue. All of these sections are separate from the novel itself, which starts with Chapter One. So if you write something in the prologue which the reader needs to know, that element is out of place, and causes the Prologue to malfunction, damaging the rest of the story.
Often when vital pieces of information are included in the prologue, it gets really long. In most cases, the prologue should be shorter than the shortest chapter of the book. Having a prologue of five thousand words or more can choke the interest out of the readers unfortunate enough to actually read it.
The easiest cure for Prologue Cancer is to cut out the prologue entirely. Disconnect it from the novel. If the novel is still alive, then the prologue can remain where it belongs, separate from the rest of the novel, giving background on the characters or the setting, but not breathing life into the novel.