NAIROBI, Kenya  A cholera epidemic is sweeping across Kenya, with 4,700 cases reported in the past month and 119 deaths in what Kenyan officials are calling “one of the worst outbreaks in a decade.”

The most stricken areas are the arid swaths of northern Kenya, which were hit this year by a devastating drought. The scant rains have meant that many people are surviving off dirty, germ-infested water, which is how cholera spreads.

The drought has also left thousands of people malnourished and weak, making them vulnerable to infectious diseases. Because of the remoteness of many of the infected areas, aid workers say they believe that the officially reported numbers of cases and deaths may vastly understate the severity of the outbreak.

“People are really sick, and coming down from the hills, and we really don’t know how bad this is yet,” Nicholas Wasunna, a humanitarian adviser for the aid group World Vision, said Friday.