正确,用原形而不是单三.
句子以Add开始,就是祈使句,祈使句的谓语动词应该用原形,因为前面省略了You should.请结合语境再看一下in many parts of the country ticks spread illness such as Lyme disease. Add to that list a new scourge: Heartland virus。我觉得应该是单三因为他用的是倒装，过来就是a new scourage adds to that list. 你觉得呢？不会的，不然要用被动式：A new scourage is added to that list.