▁only ▁in ▁humans ) ▁animal ▁models ▁do ▁not ▁exist . ▁In ▁cases ▁where ▁animal ▁models ▁were ▁not ▁available , ▁scient ists ▁have ▁sometimes ▁inf ected ▁themselves ▁or ▁others ▁to ▁determine ▁an ▁organ ism ' s ▁disease ▁causing ▁ability . ▁▁ 2 . ▁Pre - ex isting ▁theories ▁of ▁disease : ▁Before ▁a ▁path ogen ▁is ▁well - recogn ized , ▁scient ists ▁may ▁attribute ▁the ▁sympt oms ▁of ▁in fection ▁to ▁other ▁causes , ▁such ▁as ▁to xic ological , ▁psych ological , ▁or ▁gen etic ▁causes . ▁Once ▁a ▁path ogen ▁has ▁been ▁associated ▁with ▁an ▁ill ness , ▁research ers ▁have ▁reported ▁difficulty ▁disp la cing ▁these ▁pre - ex isting ▁theories . ▁▁ 3 . ▁Variable ▁path ogen icity : ▁In fection ▁with ▁path og ens ▁can ▁produce ▁varying ▁responses ▁in ▁hosts , ▁comp lic ating ▁the ▁process ▁of ▁showing ▁a ▁relationship ▁between ▁in fection ▁and ▁the ▁path ogen . ▁In ▁some ▁inf ect ious ▁dise ases , ▁the ▁sever ity ▁of ▁sympt oms ▁has ▁been ▁shown ▁to ▁be ▁dependent ▁on ▁specific ▁gen etic ▁tra its ▁of ▁the ▁host . ▁▁ 4 . ▁Organ isms ▁that ▁look ▁al ike ▁but ▁behave ▁differently : ▁In ▁some ▁cases ▁a ▁har ml ess ▁organ ism ▁exists ▁which ▁looks ▁identical ▁to ▁a ▁disease ▁causing ▁organ ism ▁with ▁a ▁mic ros cope , ▁which ▁comp licates ▁the ▁discovery ▁process . ▁▁ 5 . ▁L ack ▁of ▁research ▁effort : ▁S low ▁progress ▁has ▁been ▁attributed ▁to ▁the ▁small ▁numbers ▁of ▁research ers ▁working ▁on ▁a