A lower lobe infiltrate is a medical situation where an X-ray of the lungs shows a gray shadow on either the left or right lower lobe of the lung. The shadow can be several things, including a buildup of fluid or a bacterial infection.
One contagious infection that shows lower lobe infiltrates in a chest X-ray is Mycobacterium tuberculosis, or TB. This is an airborne bacteria that causes an active infection in approximately 2 percent of exposed patients. In patients with cystic fibrosis, a debilitating lung disease, the bacteria Pseudomonas aeruginosa can infect the airways and cause lower lobe infiltrates on an X-ray. Growing antibiotic resistance is a problem for both of these infections.