A low-income country and a high-income country face the same four kinds of hazard. What differs is how much of each lands on the people living there, and that depends on the gap in income and regulation between them. Switch each factor on or off, then widen the income gap between the two countries and watch which side the risk gathers on.
Where water is unsafe and sanitation is missing, diarrhoeal disease, cholera and parasitic infection spread, and children under five carry a large share of the deaths. Poverty removes the means to buy food, medicine, schooling and clean fuel, so each new hazard lands harder. Discrimination by gender, ethnicity, disability or caste decides who inside a country gets care, education and paid work. Companies selling tobacco, alcohol and processed food push hardest where advertising rules are weak, adding cancers, heart disease and diabetes on top of infection. Stack the four and the low-income country records lower life expectancy, higher under-five mortality and a lower HDI than the high-income one.
Safe water and sanitation, poverty and discrimination, and the marketing of harmful products explain much of the difference in health status and human development between countries, and sharing them explains the similarities.