Each of these words means "dreadful," "extremely unpleasant or disagreeable," "abominable." One can speak of "a horrid disease" or "a horrible disease" with equal meaning and emphasis. Both words are intensives, that is, terms that have a strong emotional meaning and that usually exaggerate what is actually in mind. Consequently, horrible and horrid should be used thoughtfully and sparingly. Few acts, conditions, or thoughts can truly be said to cause horror, an overwhelming and painful feeling caused by something frightfully shocking, terrifying, or revolting. Perhaps slightly less forceful words may, on occasion, be more apt: shocking, fearful, horrendous, dismaying, frightening, startling, intimidating, scary, alarming.
Dictionary of problem words and expressions. Harry Shaw. 1975.