Brucella bacteria infect cattle, goats, camels, dogs, and pigs. Transmission of the disease to humans occurs by contact with infected meat, placenta of infected animals, or ingestion of unpasteurized milk or cheese.

The illness may be chronic and persist for years. Brucellosis is rare in the United States, except in the western states and in visitors or immigrants from countries where it is prevalent (Spain, Mexico, South America, Middle East).

Approximately 100 to 200 cases occur in the United States each year. People working in occupations requiring frequent contact with animals or meat, such as slaughterhouse workers, farmers, and veterinarians, are at high risk.

Acute brucellosis may begin with mild flu-like symptoms or with fever, chills, sweating, muscle aches (myalgia), joint aches (arthralgia), and malaise. Classically, fever spikes occur every afternoon to levels around 104 degrees Farenheit. "Undulant" fever derives its name from this undulating or up-and-down fever.