The tests are outlined in the Blood Donation leaflet and will be explained at the donor session. The main ones are Hepatitis, HIV, STDs, and other reactions that might make your blood unsuitable for children or immunity compromised individuals. An example of these tests is one for cytomegalovirus (CMV), a kind of herpes virus usually producing mild common cold symptoms and a resulting immunity but may cause severe neurological damage in people with weakened immune systems and in the newborn, often transferred from domestic cats.
The reason to give blood is to assist those in need after accidents or surgery. If you think you may have an infection or have been exposed to the risk of infection then you should not donate but undergo tests at an appropriate clinic. Blood Donation should NOT be used as a method of screening for infections. If you do this you may put someone else's health at risk. Although the Blood Service screen for infections and reactions they can never be 100% sure. The system relies on donors being healthy, insofar as they are aware, and honest. See
here for more information about blood donation.