Viruses: adenovirus, coronavirus, coxsackievirus, enterovirus, influenza virus, parainfluenza virus, respiratory syncytial virus, rhinovirus

Bacteria: Bordatella pertussis, Bordatella parapertussis, Branhamella catarrhalis, Haemophilus influenzae, Streptococcus pneumoniae, atypical bacteria (e.g., Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella species)

Yeast and fungi: Blastomyces dermatitidis, Candida albicans, Candida tropicalis, Coccidioides immitis, Cryptococcus neoformans, Histoplasma capsulatum

Noninfectious triggers: asthma, air pollutants, ammonia, cannabis, tobacco, trace metals, others