Find the odd one out:
Measles, Rubella, Pneumonia, Rabies

Measles, Rubella, and Rabies are highly specific infectious diseases caused entirely by viruses. Pneumonia, on the other hand, is a general inflammatory condition of the lung tissue that is most commonly caused by bacterial infections (such as Streptococcus pneumoniae), though it can also be viral or fungal. Therefore, Pneumonia is the odd one out.