Illustration of inflamed tissue, infiltrated by a tumour (black), that is being attacked by cells of the adaptive immune system (blue). Tumours can be considered the most complex group of human diseases, causing the death of up to a quarter of populations worldwide. Benign tumours grow slowly and bulge or displace, but do not invade local vessels or adjacent organs. In contrast, malignant tumours are extremely serious because they grow rapidly and destroy adjacent structures, including blood vessels and lymphatic vessels.