Billionaire David Koch, who along with his older brother was a formidable conservative activist and donor for decades, died Friday morning at age 79. 'It is with a heavy heart that I announce the passing of my brother David,' Charles Koch said in a statement Friday. 'Anyone who worked with David surely experienced his giant personality and passion for life.' David Koch had stepped down from Koch Industries just over a year ago for health reasons. He was diagnosed with prostate cancer more than two decades ago and had been involved in donations to numerous cancer research charities and medical groups.