Each rollover, horses in Legacy have a small, mostly random chance of getting sick. They can get either a respiratory illness, or colic, and will require some treatment and time to get better.
You probably noticed I said ‘mostly random’, that's because some horses can be prone to getting respiratory illnesses or colic, and it's genetic! The genes aren't testable though, so you won't be able to tell if your horse is prone to one of the illnesses unless they get sick more frequently than other horses.
Respiratory Illnesses can be minor or moderate in severity, and moderate illnesses will require more recovery time. While a horse is sick with a respiratory illness, they will not be able to train, exercise, do their job, or show. Respiratory illnesses usually heal within 2 weeks (i.e. 1 rollover), but some of the worst cases may take 1 month.
Colic only lasts one rollover, but they will not be able to train, exercise, do their job, or show.