I believe it is a matter of public record that Coe's "glandular problem" was toxoplasmosis. Toxoplasma gondii is neither a virus nor a bacteria, it is a parasitic organism -- as are the organisms that cause malaria, although that is a very different disease. T.gondii is endemic in felines which serve as a reservoir.
It is most feared as a disease of newborns, where it can be devastating, or in immune compromised people. That latter group has included predominantly the HIV/AIDS population in recent years. Arthur Ashe died of toxoplasmosis which infected his brain in the setting of AIDS; that form of toxoplasmosis brain infection is seen almost exclusively in the immune compromised.
In the immune competent person, which presumably includes Seb Coe, the most common manifestation is lymphadenopathy (swollen lymph glands), but the disease can also involve skeletal muscle or heart muscle, among other nasty options. Certainly, even if it did not involve the muscles, I can't imagine someone trying to exercise at a world class level during the illness.
I think that if Seb Coe had a serious immune compromise (cancer, HIV, and so forth) we probably would have heard about it by now through his years in track, marriage and fatherhood, and politics. So let's (please) have no irresponsible speculation about that. As to whether hard training, aerobic or otherwise, contributed to his risk for infection, that may be the case, though I doubt we could lay the blame at any one person's training plan. Any of these systems involve hard training. Ultimately, his getting this unusual infection may have just been "one of those things . . . "