mycobacterium tuberculosis isolates from all culture - positive tb patients in seattle and king county , washington , during 20032004 were genotyped by spacer oligonucleotide typing and mycobacterial interspersed repetitive unit methods . we included patients who had an isolate that matched the outbreak strain or who had a social link to an already included patient . patient medical records were reviewed , and infectious periods were calculated . for sputum smear  positive patients , the infectious period extended from 3 months before symptom onset or the first positive smear ( whichever was earlier ) until 2 weeks after the start of appropriate tb treatment or until the patient was placed into isolation or produced consecutively negative smears . for sputum smear  negative patients , the infectious period extended from 1 month before symptom onset , the start of appropriate tb treatment , or the date that the patient was isolated ( whichever was earlier ) , until 2 weeks after the start of appropriate tb treatment or until patient isolation ( 7 ) . we interviewed patients to learn their contacts , activities , and locations frequented while they were contagious . additional contacts were found by outreach workers and a disease intervention specialist from the east - african community who was hired to work in the neighborhoods frequented by the patients . while in these neighborhoods , outreach workers and the disease intervention specialist recruited persons seen with patients or their contacts to be evaluated for tb and latent tb infection . contact activities , specifically those related to illicit drugs , were observed or self - reported . friends were defined as contacts of patients who spent time within a close - knit network of young men who exhibited similar marijuana - using behavior . other contacts were defined as the families and relatives of patients and those who were named but were not closely associated with this network . contacts received a tb evaluation including a tuberculin skin test ( tst ) to detect infection . patient 1 was first evaluated in december 2003 , when a chest radiograph suggested pulmonary tb ( i.e. , upper lobe cavitary infiltrate ) . however , only clarithromycin was prescribed , and the patient was lost to follow - up . he was again seen in an emergency room in april 2004 after the infection evolved into bilateral extensive pulmonary tb . eight patients were born in east africa ; a median of 13 years ( range 622 ) had passed since their arrival in the united states . patient 5 was a white woman who received illicit drugs from patient 1 . * unknown for 1 patient . patients were symptomatic and had findings indicating infectiousness : all had pulmonary tb , 7 had cavitary disease , and 8 had sputum that tested smear - positive for acid - fast bacilli . consecutive chest radiographs indicated progression to cavitary disease in < 75 days weeks in 3 patients and < 121 days in another patient . table 2 shows the dates of clear chest radiographs interpreted as normal and the first chest radiographs showing disease . while contagious , patients stayed in various locations , including cars , for most of the day . a single - bedroom apartment occupied by at least 1 patient while he was contagious was regularly visited by 2 other patients . numerous members of the friend network slept there on any given night , and many others would regularly visit during a 10-week period beginning in april 2004 ( figure ) . the occupants nailed boards over the apartment windows to conceal activities , primarily marijuana use , from outsiders . all reported frequent " hotboxing , " the practice of smoking marijuana with others in a vehicle with the windows closed so that exhaled smoke is repeatedly inhaled . considerable overlap in infectious periods was noted , which highlights the potential for simultaneous contact with multiple contagious patients . at least 31 ( 57% ) friend contacts spent time at the 1-bedroom apartment . after those with a past positive tst result were removed , 14 ( 64% ) of 22 screened friends and 6 ( 23% ) of 26 other contacts had a positive tst result . the risk for a positive tst result was 2.8 greater among friends than among other contacts ( 95% confidence interval = 1.36.0 ) . twenty - nine ( 54% ) friend contacts self - reported or were observed hotboxing . among the friends who reported or were observed hotboxing , 11 ( 79% ) of 14 who received a tst had a positive result . risk factors for tb include birth in a country with high tb prevalence ( 2 ) and incarceration ( 8) . although most patients in this outbreak were foreign - born and had histories of incarceration , genotyping results and epidemiologic findings suggest that tb was transmitted recently in the community rather than before immigration or during incarceration . frequent marijuana use has been reported among tb outbreak patients ( 9 ) and was the behavior linking these patients together . creative sharing of marijuana has been described recently as a factor for m. tuberculosis transmission . in australia , sharing a water pipe ( i.e. , " bong " ) was linked to transmission ( 10 ) . " shotgunning " refers to inhaling smoke from illicit drugs then exhaling it directly into another 's mouth ( 11 ) and was associated with m. tuberculosis transmission among a group of exotic dancers and their contacts ( 12 ) . this investigation noted that a similar activity , hotboxing , might have contributed to transmission . one patient with smear - positive cavitary disease reported daily hotboxing with friends , often for most of the day . in addition , marijuana smoking might induce cough , creating an ideal environment for transmission . many friends stayed and used marijuana at the single - bedroom apartment during the height of the outbreak . furthermore , by nailing boards over the windows , ventilation was limited , creating an environment similar to that of hotboxing . although progressive primary tb by nature is thought to be due to recent transmission , progressive primary tb with cavitation is uncommon ( 13 ) . however , frequent marijuana use and the setting of intense exposure may have played a role . in addition , poor nutrition and unhealthy lifestyles might have predisposed these young men to more rapid progression of disease . while no laboratory investigation to assess genetic susceptibility or strain virulence was conducted , these factors might have also contributed to the development of cases . this outbreak resembles an outbreak reported among regular patrons of a neighborhood bar ( 14 ) . both were fueled by a highly infectious source patient who spent extended amounts of time indoors with 1 group of persons who regularly used substances ( i.e. , alcohol or marijuana ) . the result in both situations was a higher than expected incidence of tb disease and latent tb infection . in the outbreak reported in this article , however , the substance of choice was illicit and further complicated the control of this outbreak . patients ' illicit drug activities promoted a reluctance to name contacts at risk and locations frequented . efforts had to revolve around meeting these young patients at times and locations convenient to the group . then after gaining the groups ' trust , outreach workers successfully found and screened contacts . many successful screenings took place on street corners and in parking spaces throughout the community . often outreach workers were successful only after spending hours driving throughout the community searching for patients and contacts . alternative strategies to name - based contact investigations may become increasingly critical to tb control as tb recedes further from the general population , yet persists within smaller guarded groups ( 15 ) .
matching mycobacterium tuberculosis isolates were noted among 11 young tuberculosis patients socially linked through illicit drug  related activities . 
 a large proportion of their friends , 14 ( 64% ) of 22 , had positive tuberculin skin - test results . 
 the behavior of " hotboxing " ( smoking marijuana inside a closed car with friends to repeatedly inhale exhaled smoke ) fueled transmission .