There are several serotypes of this bacteria that can infect a horses gut. Salmonella typhimurium appears to cause more severe symptoms than others.
Respiratory diseases are classified by their location (Upper - above the throat, or Lower - wind-pipe (trachea) and chest cavity (thorax) including the lungs - Respiratory Tract) or their causes.
Most horse owners have at some time or another had to deal with injuries of their horse, often as a result of accidents while the horse is grazing.
OCD is usually seen in young, rapidly growing horses (weanling through to 2yo), but can be occasionally seen in older horses.
While no-one wants this situation, it does occur unfortunately. Most mares that die with young foals do so very early on in the young foal's life - ie at or within a few days of foaling.