Rhodococcus equi is an important pathogen in the equine industry. It is well known for its ability to cause severe pneumonia in young foals. In addition, R. equi can cause several extrapulmonary diseases, including septic arthritis, neonatal diarrhea, abdominal lymphadenitis, abdominal and spinal cord abscesses, hepatitis, and immune-mediated diseases such as polysynovitis. R. equi can also cause sudden death in foals that appear to be healthy.

Foals are thought to become infected when they ingest or breathe in soil, dust and fecal particles harboring the bacteria within the first few days of life. While most foals are exposed to R. equi, not all foals develop disease. It remains unclear why some foals develop pneumonia caused by the pathogenic strains of R. equi while other foals do not.