The word allusion refers to a figure of speech that consists of using an external reference in a text to explain one idea but without necessarily explaining the reference itself as the author assumes the reader knows about it. This usually implies allusion relies on the background knowledge of the reader about some idea or concept that is used to reinforce the main idea of the text. Additionally, it is considered by using allusion the author links different texts and applies the concepts of one text into the other.
In the case of the text presented, which deals with the importance of vaccinations to eradicate certain diseas such as smallpox, allusion occurs in sentence 5 "So the next time someone tells you that vaccines are poisoned apples that will lead to a fate like Snow White’s, counter their argument with the facts" considering the author makes a reference to an external concept or material which Snow White's story the author uses to compare the idea of vaccines as dangerous to the poisoned apple in this story. Additionally, to this, the author just mentions the name of the story but does not explain it as it is assumed the reader knows about it, relying on the background knowledge of the reader, which is consistent with the definition and features of allusion.
