<em>The poor, plague-ridden, residential areas of the town were shut off from the rest of the world.</em>
The words <em>poor, plague-ridden and residential</em> are <em>adjectives</em>, and it is correct to separate them with commas. <em>-ridden</em> (means full of something unpleasant or bad) is a <em>suffix</em> used to form adjectives; in this case, with the <em>noun plague</em> it forms the adjective <em>plague-ridden</em>. <em>Shut off</em> is phrasal verb and doesn't need hyphen.