Mycoplasma pneumoniae is a bacterium that often causes pneumonia, especially in children and young adults. The incubation period of the bacteria is 1 to 4 weeks. The initial symptoms of an infection of mycoplasma pneumoniae are runny nose, cough, sore throat and headache. In a small percentage of the infected, the infection spreads to an actual pneumonia, symptoms that come with this include skin rash, meningitis, visual disturbances, vomiting, diarrhea, inflammation of the liver, muscle and joint pain.