Preventing diseases. Learning what causes diseases helps us to work out ways we can prevent them. For instance, no or very low amounts of important substances in our food cause what are called deficiency diseases (e.g., shortage of vitamin C causes scurvy; shortage of iodine causes goitre). Deficiency diseases are prevented by eating a balanced diet (e.g., fruit and vegetables supply vitamin C) or by adding the deficient element to food (e.g., adding iodine to table salt). Knowing that micro-organisms (germs or bugs) cause infectious diseases allows us to reduce our chances of catching those diseases. We can do this by using good hygiene to reduce our exposure to micro-organisms (e.g., washing hands before eating and after using the toilet, making sure food is prepared cleanly and cooked well), and by taking care when we handle animals that carry diseases which can infect us. Knowing which micro-organisms cause particular diseases and how body defense responses protect animals and people, allows the development of vaccines which are designed to improve the way the body can defend itself against dangerous diseases.