Bacteria thrive in reusable shopping bags. Half of the reusable bags in a recent study had bacterial growth, and 12% contained E.coli bacteria, which can cause severe intestinal illness.
Self-defense: Wash the bags with soap and water (by hand or in a washing machine). Do not carry raw foods, such as meats and produce, in reusable bags. Do not leave the empty bags in a warm car—bacterial counts rise significantly in just two hours.