Answer:
Some African countries have mortality rates over 10%. For example, according to the most recent CIA estimates, in countries like Somalia, Central African Republic, Niger and Chad, there are around 90 deaths per 1,000 live births, which is a lot.
This extremely high mortality rate affects economic growth in the long run. First of all, most economists agree that human capital is the most important form of capital, and the one that helps boost economic growth the most.
Every child who dies is potential human capital loss (and a moral tragedy as well).
If medical aid increased in those African countries, and less children died at a young age, those children would help develop the economies. However, education is also needed. If the children survive but are not well-educated, they will not be very productive in the modern economy, which is knowledge-oriented.