Just for the record, some highlights from the CIA Factbook entry on Chad:

Land use: arable land: 2.8% permanent crops: 0.02% other: 97.18% (2005) Irrigated land: 300 sq km (2003) Natural hazards: hot, dry, dusty harmattan winds occur in north; periodic droughts; locust plagues Environment – current issues: inadequate supplies of potable water; improper waste disposal in rural areas contributes to soil and water pollution; desertification Infant mortality rate: total: 91.45 deaths/1,000 live births male: 100.12 deaths/1,000 live births female: 82.43 deaths/1,000 live births (2006 est.) Major infectious diseases: degree of risk: very high food or waterborne diseases: bacterial and protozoal diarrhea, hepatitis A, and typhoid fever vectorborne disease: malaria water contact disease: schistosomiasis respiratory disease: meningococcal meningitis (2005)

Update: From a reader:

I think I would have to agree with Chad being the worst. Even one of
their most popular natural ‘beauties’, Lake Chad, comes with a
disclaimer:
“Note: Don’t wade or swim in the lake—bilharzia (parasites that cause
schistosomiasis) are present in the water.”