Yes, in most circumstances you can be a donor. Having a medical condition does not necessarily prevent a person from becoming an organ or tissue donor. The decision about whether some or all organs or tissue are suitable for transplant is made by a healthcare professional, taking into account your medical history.
In very rare cases, the organs of donors with HIV or hepatitis-C have been used to help others with the same conditions. This is only ever carried out when both parties have the condition. All donors have rigorous checks to guard against infection.