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These are organisms, such as bacteria, fungi, or viruses, whose presence in the environment creates a harmful effect, such as allergic reactions, respiratory disorders, hypersensitivity and/or infectious diseases.

According to the US Environmental Protection Agency (EPA), biological agents in indoor air are known to cause three types of human disease:

• Infections, where pathogens invade human tissues;
  
• Hypersensitivity diseases, where specific activation of the immune system causes disease; and
  
• Toxicosis, where biologically produced chemical toxins cause direct toxic effects.

In addition, exposure to conditions conducive to biological contamination -- such as those caused by dampness or water damage -- has been related to non-specific upper and lower respiratory symptoms. Evidence is available that shows that some episodes of the group of nonspecific symptoms known as "sick building syndrome" may be related to microbial contamination in buildings.