Alternaria species are known as major plant pathogens. They are also common allergens in humans, growing indoors and causing hay fever or hypersensitivity reactions that sometimes lead to asthma. There are 44 definite, well-known species, but there may be hundreds more. They are ubiquitous in the environment and are a natural part of fungal flora most everywhere. They are normal agents of decay and decomposition. The spores are airborne and found in the soil and water, as well as indoors and on objects. The club-shaped spores are single or form long chains. They can grow thick colonies which are usually black or gray. However, species of this fungal genus are often prolific producers of a variety of toxic compounds. The effects most of these compounds have on animal and plant health are not well known. The terms alternariosis and alternariatoxicosis are used for disorders in humans and animals caused by a fungus in this genus. Not all Alternaria species are pests and pathogens; some have shown promise as biocontrol agents against invasive plant species.