RESUMEN
Respiratory tract infection is the most common diseases among Iranian pilgrims during Hajj season. To understand the possibility of bacterial involvement in such infections, we screened the pilgrims' sera to determine the titer of antibodies against Mycoplasma pneuomoniae [MP], Chlamydia pneumoniae [CP] and Legionella pneumophila [LP]. Serum samples from 128 pilgrims were collected, before the trip and one month after returning home. Antibodies to MP, CP, LP were assayed using Immunoflourecent and ELISA methods. IgM antibody titre to CP did not elevated, but IgG antibody titer was increased in 34.58% [n=48] and 15.82% [n=22] of cases, indicating of recent infection. The specific antibodies to MP and LP were not increased. In pilgrims infected with an atypical respiratory pathogen, C. pneumoniae should be considered as an important causative. The true prevalence of this pathogen should be investigated since it relies on the sensitivity and specificity of currently available diagnostic methods