Okay then, wow.
If you want to do the structure of my major (legitimately inc. my subject choices) you would be choosing an Immunology major because apparently this combination only falls under that category now...to be honest I think they would be similar. For eg I took five subject from my major (four count and one is an elective) and that would allow me to qualify for both of those majors because I've added on an extra subject.
If you ask me, I don't think the two are all that separable in undergrad but let's pretend that they are...I would probably still prefer to learn something to do with the agents causing the immune system to respond rather than just the response - you will have a much fuller picture.
That said, I think a lot of people would go right into choosing an 'Immunology' major or whatever from my class because Immunology sounds more sexy than Bacteriology or Virology.
I have enjoyed both Bacteriology and Virology (probably Virology more-so)...if you're considering going on to do Honours they may enjoy it more if you do have more experience in these areas.
The reason being that when you apply, there are few strict Immunology projects in the department. You have 10 preferences and for that there are about 5 or so strictly immunological projects and then you really have to branch out into a project that combines both microbes and immunology (eg a project about the immune response to HIV, a project about the immune response to a bacterium)...if you haven't learnt about those agents and somebody else has that might make you a lesser candidate.
If you want to study this I would really suggest you just make your own major! Like if you don't want to learn about bacteria you could still learn about viruses by doing an Immunology major. Or you could do a MIIM major and then take Med/Applied Immunology as an elective like I am because you need >50 points.
I guess just ask yourself if you always enjoy learning about immunology and the cells/molecules involved or if you also like to know things about how the pathogen interacts with the host and come at it from both angles. I'd probably enjoy the fuller picture.