A substance which grants immunity against multiple or singular viruses, or sometimes diseases, such as cancer. Non-antibiotic immunogens (such as retroviruses) are, collectively, the holy grail of modern medicine.
Immunogens, generally speaking, can provide immunites before infection (such as vaccines), and also while infected. Various experimental HIV-1 immungeons are examples of this. HIV-1 is invisible to the human immune system. An HIV-1 immunogen would provide the body with the ability to recognize and fight the HIV-1 virus, even while infected.