Samples of blood, urine, sputum (lung secretions) and other body fluids or tissues are often sent to the microbiology laboratory for testing. These tests are designed to determine whether or not germs (bacteria, viruses, fungi, and other microorganisms) are present and causing illness. The samples are studied under the microscope and then are cultured. Culturing means that the samples are prepared and stored in a way that promotes germ growth. Often within 48-72 hours the laboratory is able to determine which germs are causing the problem and which antibiotics are most effective against the germs. Sometimes the germs do not grow in the culture even though the patient has an obvious infection.