The term is usually limited to known pathogens, but also sometimes includes agents such as simian foamy virus which are not known to cause disease.
Donors of blood are also screened for signs and symptoms of disease and for activities that might put them at risk for infection.
Human immunodeficiency virus (HIV) leads to the most well-known of the transfusion transmitted diseases, acquired immune deficiency syndrome (AIDS).
Most of the viral diseases are spread by either sexual contact or by contact with blood, usually either drug use, accidental needle injuries among health care workers, unsterilized tattoo and body piercing equipment, or through a blood transfusion or transplant.
[citation needed] Whether a donor is considered to be at "too high" of a risk for a disease to be allowed to donate is sometimes controversial, especially for sexual contact.