Microbes – bacteria, archaea, fungi, algae, protozoa and viruses – affect many aspects of life on earth. Some of them are vital to our health, and some cause severe diseases. Many species of microorganisms live on mucous membranes where they form a normal flora. None of these tissues, however, is infected. Healthy microbial growth is vital on the skin and mucous membrane as it protects us from pathogens. Most of the microbes we come across in our daily lives do not cause diseases. However, a microbe that is normally harmless might cause an infection to human with reduced immunity for instance, because of illness. For this reason, hospitals have high standards for cleanliness and visitors must take care of proper hand hygiene. Also in crowded public places like schools, traffic hubs and mass gatherings, special attention needs to be put on to reduce the spread of microbes that cause infectious diseases.