This is a two part question. First part: a few diseases require or allow an article (the flu, the measles, the shingles), but most do not permit it (polio, diarrhea, cancer, COPD). You can test this in the sentence: "Last year I had [the][disease]". What determines when an article is not permitted or required before the name of a disease?
Second part: If someone does use an article in front of a disease that normally does not need one-- "In January my cousin got the Covid"--is this more than just a solecism? It might not be an error, but rather a style of speech. But it implies, to me, that the speaker is uneducated or from a lower economic class.