Rhodococcus equi is a gram-positive, facultative intracellular pathogenic bacteria that infects domesticated animals (horses, sheep, pigs, goats, cattle, dogs and cats); but also immunocompromised people, such as AIDS patients and transplant recipients are at risk of an infection with R. equi. However, R. equi is a severe problem in horse breeding since it is the main cause of lethal chronic bronchopneumonia in fols. Infection is observed for fols of an age between three weeks and five months. Typically, infections are transmitted through grazing and thereby inhalation of contaminated soil. Within excrements and soil R. equi is durable and is active at temperatures over 10°C.