The WHO produces a report on causes of death wordwide[0]. Heart disease and stroke are the biggest killers, but these are, like cancer, really quite complex. Diarrhoeal diseases and respiratory infections kill a ridiculous number of people, but these are largely solved problems in the western world. Organisations like the Gates Foundation are doing great work exporting what we know about these diseases to the developing world, but governments do a pitiful amount. A number of chronic lifestyle diseases are rising: lung/throat cancer (due to smoking) and diabetes (due to eating sugar). My suggestion is that we should prioritise disease research and treatment based on how many people are dying and how easily those problems could be solved. But because funding bodies are all human, and all in the developed world, the disease they most fear is cancer.
Regarding food security, there is technically enough food produced in the world to feed everyone, if we could solve the distribution and storage problems and prevent rich governments' protectionist interference in agricultural markets and corruption in developing nations' governments. But a large amount of that wastage is also not really suitable for human nutrition. Probably the fastest way to alleviate hunger is to enable each part of the world to feed itself with food produced nearby. Because we've reached the limits of arable land in much of the world, this means making more land suitable for food production, finding new ways to provide water, and making crops be able to grow better in different conditions. The FAO's State of Food Insecurity report[1] is a good summary.