Brucella bacteria, illustration. Brucella is a Gram negative, non-sporing, aerobic bacillus (rod- shaped bacteria). It is primarily a pathogen of farm animals but also causes recurrent fevers in humans. B. abortus is effectively the sole causative agent of human disease in the UK, producing symptoms of fatigue, fever, anorexia, depression and muscle and joint pain. Contraction of human disease may occur through unpasteurised milk, but is more likely to be restricted to persons working in close contact with farm animals (veterinary surgeons, farm workers etc.).