Sahel drought helped to cement such seasonal migration patterns. Men from a community will often travel together to the same towns on each year, many to the same areas that their fathers had traveled. For many in rural communities that pursue subsistence farming, that provides most of their yearly cash income and is thus a crucial element of the rural economy, but it is not counted in the formal economy of Niger. Cash earned is partially spent abroad for necessities such as clothing, carried back at the end of the season, or sent via friends and clan or ethnic networks. A 2008 study found that not only most migrant workers never use of banks or money transfer systems but also the Exode period is often a time that men will take out informal loans against their expected seasonal earnings. Disease spread Men on Exode may also bring back sexually transmitted diseases from their season abroad. That has been flagged as a potential vector for HIV/AIDS to enter Niger, which currently has one of the lowest infection rates in the world. Measles outbreaks (largely among young children) still occur in Niger, in part because of the low vaccination rate and in part due from the transhumance seasonal migration of semi-nomadic herding populations. Sporadic outbreaks in Nigerien communities were found to have occurred beginning at the end of the rainy season, when many rural populations begin seasonal migration pattern, with traveling children often missing their vital second immunization booster against the disease. Ethnic differentiation Zarma-Songhai men often travel to Ghana and Burkina Faso, retracing a pattern of migration recorded from at least the 17th century, when Zarma soldiers were recruited to fight for the small kingdoms in what is now northern Ghana and southern Burkina Faso. The trade networks that resulted from the migration survived throughout the colonial period, and they also allowed Djerma a way of escaping to British-controlled Gold Coast Colony during times of particularly onerous French forced labour under the Indigénat as well as in times of drought in the 1910s, 1930s and 1940s. The example of the Zarrma-Songhai of Niger's migration to the former Gold Coast Colony is memorably portrayed by French filmmaker Jean Rouch in his film "Jaguar" (1954-1955). For the film and accompanying academic study, Rouch joins an urban educated Songhai (Damouré Zika), a Sorko fisherman (Illo Gaoudel), and a Fula herdsman (Lam Ibrahima Dia) who