Every spring an insidious mental disease afflicts a small segment of the American population.

Its symptoms are megalomania, delusions of grandeur and the distortion of rational thought. It strikes selectively at people who own 3-year-old thoroughbreds.

It's called Derby fever, and its ultimate effect is to make people who have bad horses believe that they are good horses.

This distinction ought to be an easy one to make. For example, if a horse runs against moderate opposition and loses by 10 or 20 lengths without a visible excuse, a rational man might conclude that he is a relatively untalented animal. But not when Derby fever strikes.

The case history of Bert Sonnier is typical. The trainer had high hopes for his Derby nominee. Execution's Reason, at the start of the season, but the colt ran poorly in Florida. Undaunted, Sonnier sent him to run in the Arkansas Derby against a weak field. Execution's Reason finished a dismal eighth.

Sonnier could not come to the obvious conclusion after this performance. He explained: "The rider, Jim McKnight, said the colt was getting dirt in his face and didn't seem to care for it. He wants to be closer to the pace and perhaps he resented it when he was taken back." Therefore, Sonnier has brought his colt to Churchill Downs, where he may think there is no dirt on the track.

Derby fever struck hard at Mr. and Mrs. Russell Braun, owner of a colt named Swift Dela Swift. With visions of roses in their heads, they entered him in an allowance race at Caliente, a low-grade track in Mexico. Swift Dela Swift lost, but that didn't jolt the owner's Derby aspirations a bit.

Mrs. Braun told the Daily Racing Form, "He drew an outside post, went into the air at the break, was bumped on the first turn and went wide into the stretch, yet still finished second." She considered this was the most heroic defeat since the Alamo.

Thus do owners for improbable horses like Tonka Wakhan and El Gato Grande keep aiming them for the Derby. They believe their horses would be proving themselves as the new Secretariat if they didn't keep getting bumped, or moving prematurely, or moving too late, or coming down with fevers, or disliking the hot weather in the south, or disliking the cold weather in the south, or disliking the cold weather in the north, or running over race tracks that are too "cuppy."

If an owner can't find an excuse for a past defeat, he can always justify coming to Churchill Downs on the grounds that his horse will like the Derby distance of 1 3/4 miles. People who watched Plugged Nickle win the Wood Memorial Stakes on television Saturday may not have been aware that a horse named Inland Voyager was in the field; he was too far behind. But since the colt was running last early in the race (and could thus move nowhere but up from there) he rallied to finish a distant fifth. That was enough for trainer Lou Rondinello and owner John Galbreath. Inland Voyager is coming to the Derby.

It is tempting to dismiss everything that owners and trainers say before the Derby as absurdities induced by Derby fever. But on rare occasions there may be a legitimate reason why a horse has not shown the full extent of his ability.

A handicapper has to distinguish rationalizations from legitimate excuses, and that will be an important task today.

Two of the principals in the Blue Grass Stakes at Keeneland, the last major prep for the Derby, are horses who have a lot a ability and a lot of excuses.

Rockhill Native, was last year's champion 2-year-old, but he was erratic at best in Florida this winter. Trainer Herb Stevens blamed the Hialeah racing surface, saying it was too deep and "cuppy" for Rockhill Native. This is one of the oldest and tritest excuses in the book, but Rockhill Native did improve sharply when he won an allowance race at Keeneland last week. Maybe, just maybe, he really did not like the Hialeah track.

Gold Stage ran well as a 2-year-old and whipped Plugged Nickle in his first start of the season, but then ran very poorly for the rest of his Florida campaign. Trainer Bill Curtis offered a couple of feeble-sounding rationalizations: Gold Stage didn't like the Florida heat. A virus set back his training. But Gold Stage has come back to life in Kentucky, turning in a sensational workout for the Blue Grass.

One of these colts may definitely prove that he is back in top shape by winning today. But the losers need not despair. They will surely have an excuse.