two american scientists ( patients 1 and 2 ) lived and worked in the village of bandafassi in southeastern senegal in august 2008 while performing a mosquito - sampling project in surrounding villages . patients 1 and 2 were men ( 36 and 27 years of age , respectively ) , and both had received the yellow fever 17d vaccine before their travel to senegal . during their project , mosquitoes in the evenings while they worked . patients 1 and 2 left bandafassi on august 21 , stayed in dakar for 2 days , and then returned to their homes in northern colorado on august 24 . symptoms in patient 1 began on august 30 and consisted of swollen ankles , a maculopapular rash on his torso , and extreme fatigue and headache , but no fever was recorded . on august 31 , he experienced the same symptoms and light - headedness and chills , wrist and ankle arthalgia , and symptoms of prostatitis ( perineal pain and mild dysuria ) . fatigue and rash decreased on september 1 ; only residual wrist arthralgia , headache , and prostatic symptoms persisted . on september 2 , two aphthous ulcers appeared on his lip . on september 3 , he and his wife observed signs of hematospermia ( red  brown fluid in his ejaculate ) that lasted until september 7 . patient 2 experienced his symptoms during august 29september 1 , which included a maculopapular rash on his torso , extreme fatigue , headache , and swelling and arthralgia in his wrists , knees , and ankles . acute - phase blood samples were obtained from both patients on september 2 . in patient 3 ( a nurse and the wife of patient 1 ) similar clinical symptoms developed on september 3 , including malaise , chills , extreme headache , photophobia , and muscle pain that continued through september 6 . she did not have detectable fever . on september 7 , a maculopapular rash developed on her torso ( figure ) that expanded to her neck and thighs on the following day , and an aphthous ulcer developed on her inside lip . on september 8 , patient 3 had an acute - phase blood sample drawn on september 8 . on september 11 , she visited her primary care physician , who performed a complete blood count test and studies of hepatic function ; all results were within reference ranges . patient 2 reported wrist arthralgia for 1 month after his acute illness , and patients 1 and 3 have had recurring wrist or thumb joint arthralgia since their acute illness . convalescent - phase blood samples were drawn on september 22 from patients 1 and 2 and on september 26 from patient 3 . acute - phase and convalescent - phase paired serum specimens from the 3 patients were tested independently by several different laboratories . results of virus isolation were negative for all samples when tested in vero and aedes albopictus mosquito ( c6/36 ) cell cultures and by intracerebral inoculation of acute - phase serum of patient 3 into suckling mice . pcrs with 16 different sets of arbovirus - specific primers did not detect arboviral rna in any of the samples . serologic analyses ( table a1 ) of samples from patients 1 and 2 showed matching results . hemagglutination inhibition antibody titers and virus neutralizing titers were highly elevated above background levels for zikv and yellow fever virus ( yfv ) compared with other viruses tested . these titers most often increased in the time between obtaining acute - phase and convalescent - phase serum samples . hemagglutination inhibition , complement fixation , and virus neutralizing titers against zikv alone developed only in the convalescent - phase sample of patient 3 . evidence suggests that patients 1 and 2 were infected with zikv , probably in southeastern senegal , by bites from infected mosquitoes . the village of bandafassi is located in a disease - endemic area where zikv has been isolated from humans , nonhuman primates , and mosquitoes ( 4,5 ) . serologic results suggest an anamnestic response to zikv infection , likely stemming from their vaccination with yfv . the time between infection and the onset of clinical manifestations can be inferred to be > 9 days , given the patients travel history . their clinical symptoms are consistent with reported symptoms of zikv - associated disease ( 3,69 ) . exceptions are aphthous ulcers in patient 1 ( also reported by patient 3 ) , prostatitis , and hematospermia . whether these exceptions are typical but unreported symptoms or clinical anomalies is not clear . patient 3 had never traveled to africa or asia and had not left the united states since 2007 . circumstantial evidence suggests direct person - to - person , possibly sexual , transmission of the virus . temperatures and mosquito fauna on the northern front range in colorado when transmission occurred do not match known mosquito transmission dynamics of zikv by tropical aedes species . patient 3 had zikv disease 9 days after the return of her husband from senegal . however , the extrinsic incubation period of zikv in ae . area temperatures during the week of return of patient 1 fluctuated between 10c and 31c , only ae . vexans mosquitoes of the subgenus aedimorphus are commonly captured on the northern colorado front range , and known tropical zikv vectors are mostly from the subgenus stegomyia ( 4 ) . mosquito sampling around the home of patients 1 and 3 at the time yielded only 7 ae . furthermore , patients 1 and 3 reported having vaginal sexual intercourse in the days after patient 1 returned home but before the onset of his clinical illness . it is reasonable to suspect that infected semen may have passed from patient 1 to patient 3 during coitus . another possibility is that direct contact and exchange of other bodily fluids , such as saliva , could have resulted in zikv transmission , but illness did not develop in the 4 children of patients 1 and 3 during this time . to the best of our knowledge , human sexual transmission of an arbovirus has not been documented . however , japanese encephalitis virus was discharged into the semen of experimentally infected boars and could infect female pigs by artificial insemination ( 11 ) . furthermore , west nile virus rna and st . louis encephalitis virus antigen have been detected in urine of humans ( 12,13 ) , and viruria has occurred in hamsters infected with west nile virus ( 14 ) and modoc virus ( 15 ) . if sexual transmission could be verified in subsequent studies , this would have major implications toward the epidemiology of zikv and possibly other arthropod - borne flaviviruses .