I recall reading some time ago an interesting theory on stable form. It was suggested that there is something wrong in every yard 99.9% of the time, mostly undetected viruses where, like humans, some horses are affected and some are not. Some end up with the equivalent of a slight cold, others it hits hard.
A huge anount of food goes in and out of the yard. Some of it will carry contamination at some level. That contamination, like the virus, might affect 2% of the horses or 72% of the horses.
Trachea washes, blood tests, etc., generally show nothing, or show something else that throws treatment off track. So, when yards hit a rich vein where almost everything runs to form, it is nothing more than the stars aligning for a couple of weeks when viruses and contamination have somehow missed the yard for that short spell. Also, yards in the big training centres are more susceptible to ill health simply on the crowd-infection basis – passing in the street, using the same gallops etc.
There might be nothing in it but it struck me as highly plausible and still does.