There are over 200,000 species of fungi that are important for medical purposes. These microscopic organisms are only about ten micrometres long, ten times smaller than a strand of hair. They mostly live in equilibrium with humans. Some of these species cohabit with bacteria to form the gut microbiota. They represent 10% of the intestinal flora and play an essential role in our health. However, about 100 of these microorganisms can affect humans, particularly people with weakened immune systems. Every year, fungal infections kill more than 1.5 million people worldwide. Although infections caused by fungi and yeasts are pretty common and mostly benign, they alter the hydration, pH, nutrient concentrations or microbial composition of the skin and mucous membranes. Their development into mycosis is favoured in humid areas, mainly where there is much sweating: the groin, armpits, interdigital spaces, wounds and mucous membranes. The infection can become invasive when it is chronic or reaches the bloodstream, mainly affecting people with an immune deficiency or infants. It has been on the increase for many years in hospitals.