There are too many rules about commas to repeat here, so you may wish to check the references cited. If after checking the rules you are still uncertain of whether or not to use a comma in a given sentence, read the sentence aloud and see if it sounds better with or without a pause, and then add or omit the comma. The disease began in Sweden in 1972 and since then it has spread. It can never function as a colon. As a weak period, the semicolon joins independent statements that the writer wishes to bring together more closely than a period allows.
Students tend to overuse semicolons, which have specific, limited functions.
A fragment is a part of a sentence set off as if it were a complete sentence. A sentence fragment is often an afterthought that really is an explanation or other addition to the previous sentence. With appropriate punctuation, a fragment can usually be connected to the previous sentence. Although clutch sizes vary with feeding in other species of birds. A possibility which is impossible for the first study.