An example of a scientific theory that underwent many changes is the germ theory of disease. In ancient times, people believed that diseases were caused by the gods, or by curses, or by improper behavior. Germs (microorganisms) were unknown, because germs are too small to see. With the invention of the microscope, germs were discovered, and the germ theory of disease was proposed. Thanks to the germ theory of disease, many diseases can now be cured. However, the germ theory of disease had to be modified, because some diseases are not caused by germs. The flu and scurvy are examples of diseases that are not caused by bacteria, but viruses or poor nutrition. Scientists modified the germ theory of disease, so that today we would state that theory as "Some diseases are caused by germs".