Some injuries and possibly infections predispose to cancer. Radiation and excess sunshine can damage DNA in cells. Social stress does not cause cancer but in cats (and rats), increases the speed at which a cancer grows, especially cancers caused by viruses. Stress appears to inhibit the numbers of and efficiency of natural killer cells, the cells that normally spot cancer cells and coordinate their destruction. Hormonal factors are related to cancer, which is why the most common cancers affect the reproductive systems and can be reduced or eliminated by early neutering. Some cancers evolved successful techniques that permit wider spread throughout the body. These malignant tumours produce chemicals that actively suppress a pet's immune system.