Look at this about piggie pheumonia:

Pneumonia is one of the most common bacterial diseases of pet guinea pigs. A number of potential disease-causing bacteria may inhabit the respiratory tracts of otherwise normal guinea pigs. Stress, inadequate diet, and improper home care often predispose a pet guinea pig to respiratory infection. Signs of pneumonia may include labored or rapid breathing, discharge from eyes and nostrils, lethargy and inappetence. Some animals show no signs at all before dying suddenly. Middle and inner ear infections occasionally result from respiratory disease in guinea pigs. Additional signs may include incoordination, tilting of the head, circling to one side, and rolling. A veterinarian must be consulted about this serious bacterial infection. Aggressive antibiotic therapy by injection and appropriate supportive care are necessary. Bacterial culture and antibiotic sensitivity testing help the veterinarian select an appropriate antibiotic. Unfortunately, even though the signs of in-fection can be relieved, the causative bacteria cannot be eliminated. Rabbits and rats harbor at least one of the bacteria known to cause pneumonia in guinea pigs. Therefore, it is wise not to house these animals with or near guinea pigs.
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