In context you can just spell "PN."
PNEUMONIA = "PN"
Some people do a version the sign "lungs" using a distressed facial expression. This is also the same sign as you might use for one of the versions of ASTHMA:
PNEUMONIA or ASTHMA (relies on context or is combined with some amount of fingerspelling)
PNEUMONIA (initialized version)
Many people initialize the sign for LUNGS with the letter "P" to mean "pneumonia."
Notes: