I would bet money that shroom is getting the flu virus but not the immune reaction. This collection of bio material is what holds you back when you finally become more reactive. Each strain collected over the years must be dealt with; this is the whole theory behind the bio-weapon Lyme disease. Collect bio material while the immune system can no longer show reactions.
Same as fluoride poisoning, it allows the collection of heavy metals without immune response. This is why it is used for people who claim to have parasites, adding a ton of fluoride in the form of Prozac will dump down your immune response again and allow your zoo to grow as intended...