used much during the war. The killed, louse-grown vaccine was difficult to manufacture in large enough quantities, and the French vaccine was not believed to be safe enough for use. The Germans worked to develop their own live vaccine from the urine of typhus victims. While developing a live vaccine, Germany used live Rickettsia prowazekii to test multiple possible vaccines' capabilities. They gave live Rickettsia prowazekii to concentration camp prisoners, using them as a control group for the vaccine tests. The use of DDT as an effective means of killing lice, the main carrier of typhus, was discovered in Naples. Society and culture Biological weapon Typhus was one of more than a dozen agents that the United States researched as potential biological weapons before President Richard Nixon suspended all non-defensive aspects of the U.S. biological weapons program in 1969. Poverty and displacement The CDC lists the following areas as active foci of human epidemic typhus: Andean regions of South America, some parts of Africa; on the other hand, the CDC only recognizes an active enzootic cycle in the United States involving flying squirrels (CDC). Though epidemic typhus is commonly thought to be restricted to areas of the developing world, serological examination of homeless persons in Houston found evidence for exposure to the bacterial pathogens that cause epidemic typhus and murine typhus. A study involving 930 homeless people in Marseille, France, found high rates of seroprevalence to R. prowazekii and a high prevalence of louse-borne infections in the homeless. Typhus has been increasingly discovered in homeless populations in developed nations. Typhus among homeless populations is especially prevalent as these populations tend to migrate across states and countries, spreading the risk of infection with their movement. The same risk applies to refugees, who travel across country lines, often living in close proximity and unable to maintain necessary hygienic standards to avoid being at risk for catching lice possibly infected with typhus. Because the typhus-infected lice live in clothing, the prevalence of typhus is also affected by weather, humidity, poverty and lack of hygiene. Lice, and therefore typhus, are more prevalent during colder months, especially winter and early spring. In these seasons, people tend to wear multiple layers of clothing, giving lice more places to go unnoticed by their hosts. This is particularly a problem for poverty-stricken populations as they often do not have multiple sets of clothing, preventing them from practicing good