B. pseudomallei (formerly Pseudomonas pseudomallei) is a motile, aerobic, non-spore-forming, saprophytic, gram-negative bacillus. The organism is distributed widely in contaminated soil and water in tropical regions, especially in Southeast Asia where it is endemic, and is spread through direct contact with the contaminated source. Human cases have resulted from sexual contact and I.V. drug use. It has been observed in immigrants, military personnel, and travelers. The incubation period can vary from days to months to years; after an aerosol attack, the incubation period ranges from 10 to 14 days.

Bacteria that enter skin through a laceration or abrasion cause a local infection, ulceration, swollen lymph glands, and increased mucus production (when they enter through mucous membranes). Bacteria that enter the respiratory tract can cause pulmonary infections (pneumonia, pulmonary abscesses, and pleural effusions) and, possibly, cutaneous abscesses that may take months to appear. When bacteria enter the bloodstream, chronically ill patients (such as those with human immunodeficiency virus or diabetes) develop respiratory distress, headaches, fever, diarrhea, pus-filled lesions on the skin, and abscesses throughout the body.

The chronic form of melioidosis involves multiple abscesses that may affect the liver, spleen, skin, or muscles. Melioidosis can also become reactive many years after the primary infection.

Complications from melioidosis include septicemia, osteomyelitis, meningitis, and brain, liver, or splenic abscess. Prior to the use of antibiotics, untreated patients with septicemia had a mortality rate as high as 95%. Mortality remains higher than 50% for septicemic disease and reaches 20% for localized disease despite treatment; the overall mortality rate is 40%.