Underlying cause of illness

This is an easy one. The underlying cause of the majority of illness is poverty. Poverty has a direct relationship to infections disease. Malaria, tuberculosis, etc.have been repeatedly shown to be linked to lack of clean water, sanitation, food, money, and access to medicines. And disease of course, serves to further impoverish the world's poor such that the numbers are ever rising.

1.1 billion people have inadequate access to water
2.6 billion lack basic sanitation
80% of humanity currently has little or no housing.
50% - one of every two people on the planet - are currenly dying of malnutrician and starvation.
(Figures here are garnered from several UN, Unicef, UNHCR, and Amnesty reports.)

There are 2.2 billion children in the word, 1 billion of whom (1/2 of all the world's children) live in poverty. As a result 30,000 children die each day due to poverty (insufficient food, medicine, homes, clean water, etc.) To quote from a recent UN study of those children:  they “die quietly in some of the poorest villages on earth, far removed from the scrutiny and the conscience of the world. Being meek and weak in life makes these dying multitudes even more invisible in death.”

According to UNICEF roughly 11,000,000 children under five years of age die for reasons directly tracable to being poor. Yet much of this could be easily prevented: Every child in the world could have received basic immunization for the next fifty years for the cost of the monies spent by the United States on its most recent war in the Middle East.

In developed countries the rising number of poor highly correlate to the rising numbers of heart, digestive, and similar disease. The cost of helping?

The 200 wealthiest people in the world (most of whom live in the United States) have between them more than $1.5 trillion in assets - more than ten times the combined income of all of the people in the 43 least developed countries [ibid]. These wealthy could in one fell swoop eleviate most poverty and illiteracy if they wished to share their fortunes. (Update: since I wrote this, the wealth of these 200 people is in excess of $2.3 trillion USD.)

US military spending is roughly $50,000,000 per hour. Consider: Malaria could be completely eliminated from the world for the cost of a mere few hours worth of this expense. Every person in Africa effected by aids could have pain medication, life extending drugs, and clean clothing for just a few more hours of military indulgence.

The world's 100 wealthiest corporations, the majority of which are US owned, hold and control more fiscal resource than 2/3 of humanity.
Malaria, dissentary, most of the aids epidemic, and most major diseases could be halted or even eliminated by a mere fraction of this resource.

The cause of most disease is there not merely poverty, it is the unwillingness of those fully capable of eleviating poverty and consequent disease, from doing so.