A blood culture takes blood from the patient and places small quantities of it in a sterile mixture of blood and an agar gelatine. As the patient's blood will not replicate in such conditions, if there are any bacteria or fungi in the blood, they will grow in colonies in the agar which will be visible to the naked eye. Those bacteria and fungi can then be viewed under a microscope to identify them. As such, blood cultures can provide a definitive diagnosis for many forms of infectious diseases.