In the United States the median age at which colon cancer strikes is 69 for men and 73 for women. In Chad the average life expectancy at birth is about 50. Children who survive childbirth — and then malnutrition and diarrhea — are likely to die of pneumonia, tuberculosis, influenza, malaria, AIDS or even traffic accidents long before their cells accumulate the mutations that cause colon cancer.

In fact, cancers of any kind don’t make the top 15 causes of death in Chad — or in Somalia, the Central African Republic and other places where the average life span peaks in the low to mid-50s. Many people do die from cancer, and their numbers are multiplied by rapidly growing populations and a lack of medical care. But first come all those other threats.

How different this is from the United States, where oncologists are working to rid a 91-year-old former president of metastatic melanoma, one of the deadliest cancers. One of Jimmy Carter’s drugs, a new immunotherapy agent called Keytruda, has been priced at $12,500 a month, in addition to the cost of his surgery and treatment with computer-guided radiation beams.

Mr. Carter, a religious man, says he is prepared to meet his maker. But he is among the fortunate who first have the luxury of exhausting the most expensive remedies medicine has to offer.