Grass sickness is a nasty disease that mostly affects animals in northern Europe, especially Great Britain. It affects all equines, not just horses. (A similar disease effects rabbits and hares).
Grass sickness generally affects younger horses between 2 and 7 years. Older horses appear to develop resistance to the disease. It almost always affects horses on pasture, hence the name.
We have no idea what causes it. It might be a deficiency (it seems to be associated with high nitrogen contact of soil) or, the current lead theory, a soil bacterium called Clostridium botulinum. It's not contagious, but several cases may occur if multiple horses are in the same pasture.
It causes partial or complete paralysis of the digestive tract and is almost universally fatal in the acute form. The symptoms of acute grass sickness include constipation, a distended stomach and partially digested food coming out of the nose - and the horse usually dies within a couple of days.
Subacute grass sickness is less severe, but is still fatal. Chronic grass sickness, which has symptoms that include rapid weight loss, is survivable and some cases can be treated with nursing and special high energy feed.
Grass sickness is horrible and there's no way to prevent it other than not using pasture associated with the disease for grazing horses in spring and summer. (US people, yes, grass sickness is all but unknown in the US, with only a very occasional case diagnosed and as it can sometimes be confused with other forms of colic...)