Is Atypical Pneumonia Contagious?
Yes, atypical pneumonia is contagious, particularly infections caused by Mycoplasma pneumoniae and Chlamydia pneumoniae, which spread through respiratory droplets from person to person. 1, 2
Transmission Patterns by Pathogen
The contagiousness of atypical pneumonia depends on the specific causative organism:
Highly Contagious Pathogens
Mycoplasma pneumoniae is transmitted person-to-person via respiratory droplets and accounts for 13-37% of all outpatient pneumonia episodes, with epidemics occurring approximately every 4-5 years. 1 This cyclical epidemic pattern demonstrates its highly contagious nature in community settings. 3
Chlamydia pneumoniae spreads through respiratory transmission and is reported in up to 17% of outpatients with community-acquired pneumonia. 1 Like M. pneumoniae, it transmits readily between individuals through close contact. 2
Zoonotic (Not Person-to-Person) Pathogens
Legionella species are acquired from environmental water sources (cooling towers, plumbing systems) and are not transmitted person-to-person. 4 Rates vary from 0.7% to 13% of outpatients. 1
Coxiella burnetii (Q fever) is a zoonotic pathogen transmitted from animals, not between humans. 5, 4
Chlamydia psittaci (psittacosis) is acquired from infected birds and is not contagious between people. 5, 4
Clinical Implications for Infection Control
When to Consider Contagiousness
School-aged children and adolescents with M. pneumoniae (8-16% of hospitalizations in this age group) represent a particularly contagious population due to close contact in educational settings. 1
Mixed infections involving both typical bacterial and atypical pathogens occur in 3-40% of cases, which may affect transmission dynamics. 1
Key Pitfall to Avoid
Do not assume all atypical pneumonias are contagious. 4 Zoonotic atypical pneumonias (Legionella, Q fever, psittacosis) can be eliminated from consideration with a negative environmental or animal contact history, as these do not spread person-to-person. 4
Practical Guidance
For M. pneumoniae and C. pneumoniae: Standard respiratory precautions are appropriate, as these spread through respiratory droplets similar to typical bacterial pneumonia. 2
For Legionella: No isolation precautions are needed, as this is environmentally acquired and not transmitted between people. 4
For suspected zoonotic causes: Focus on identifying exposure history (birds for psittacosis, farm animals for Q fever) rather than implementing contact precautions. 4