According to this article 65% of supermarket chickens contains a bug called campylobacter. The bacteria causes around 55,000 people a year to fall ill every year and causes symptoms such as diarrhoea, cramping and stomach pain which usually last for about a week. People with weakened immune symptoms can be at risk of the bacteria spreading to the bloodstream and causing a life threatening infection. The illness can however, be prevented by properly cooking food. Surprisingly Free range and organic chicken are actually more likely to carry the bug than battery birds.