Fungi (the singular is fungus) are plant-like organisms that do not have chlorophyll (the green pigment most plants have, which allow them to convert sunlight into energy).
Lacking chlorophyll, they do not require sunlight for growth, and prosper under conditions like those enjoyed by bacteria (warmth, relative darkness, moisture). Some fungi are microscopic (e.g., yeasts), but others are readily seen with the naked eye (e.g., moulds and mildews).
One of the difficulties with moulds is that they form fungal spores as part of their normal life cycle. These fungal spores, while not as difficult to kill as bacterial spores, can survive indefinitely in dry, dusty conditions.
Most people’s immune system can resist infection from fungi, so that fungal infections are rare. However, people who do have a completely adequate immune system can become infected with some types of fungi, primarily yeasts and moulds.
The disease results from breathing in the spores. It is often associated with building renovations.
Mildews will grow anywhere there is enough moisture. While mildew may look and smell offensive, it does not infect people. They are, therefore, not an infection control concern.