i.e., inhalation of one organism will yield disease in 50% of the population. This is an extremely low infectious dose (only 1-10 organisms required), making C. burnetii one of the most infectious known organisms. Disease occurs in two stages: an acute stage that presents with headaches, chills, and respiratory symptoms, and an insidious chronic stage. While most infections clear up spontaneously, treatment with tetracycline or doxycycline appears to reduce the symptomatic duration and reduce the likelihood of chronic infection. A combination of erythromycin and rifampin is highly effective in curing the disease, and vaccination with Q-VAX vaccine (CSL) is effective for prevention of it. The bacteria use a type IVB secretion system known as Icm/Dot (intracellular multiplication / defect in organelle trafficking genes) to inject over 100 effector proteins into the host. These effectors increase the bacteria's ability to survive and grow inside the host cell by modulating many host cell pathways, including blocking cell death, inhibiting immune reactions, and altering vesicle trafficking. In Legionella pneumophila, which uses the same secretion system and also injects effectors, survival is enhanced because these proteins interfere with fusion of the bacteria-containing vacuole with the host's degradation endosomes. Use as a biological weapon The United States ended its biological warfare program in 1969. When it did, C. burnetii was one of seven agents it had standardized as biological weapons. Genomics At least ten completely sequenced genomes of Coxiella burnetii strains exist, which contain about 2.1 Mbp of DNA each and encode around 2,100 open reading frames; 746 (or about 35%) of these genes have no known function. In bacteria small regulatory RNAs are activated during stress and virulence conditions. Coxiella burnetii small RNAs (CbSRs 1, 11, 12, and 14) are encoded within intergenic region (IGR). CbSRs 2, 3, 4 and 9 are located antisense to identified ORFs. The CbSRs are up-regulated during intracellular growth in host cells. Additional images References External links Coxiella burnetii genomes and related information at PATRIC, a Bioinformatics Resource Center funded by NIAID Legionellales Biological weapons Gram-negative bacteria Bacteria described in 1939