The organism does not typically cause disease in other species of animals. The pus and nasal secretions appear to have some protective effect such that the organism has been reported to survive for weeks to months outside the horse's body in a contaminated environment. The disease is highly contagious and can be transmitted by direct contact with nasal secretions or pus from a draining abscess. Both of these methods of disease transmission are very significant and will be discussed later in this article. Members of the equine species of all ages can be affected, but the disease is most common and most severe in young horses.
Many foals have some degree of immunity to the disease that they acquire from their dams' colostrum.
Many horses will not develop sound immunity against the disease until after they have suffered from the disease. Another important factor is the addition of a new animal. More on that later. Once the Streptococcus equi organism is transmitted to a horse via the mouth or nose, the organism attaches itself to the tonsil and adjacent lymph nodes. The lymph nodes are connected throughout the body by channels called the lymphatics, so once the organism has invaded one lymph node, it has a highway, so to speak, of lymphatics to spread around to other lymph nodes.