Rather than the non manifestation of symptoms reflecting people not getting sick, is it not possible you do still get illnesses / viruses but the immune system being suppressed or compromised just means they are not symptomatic?
I only ask because I remember some old friends of mine who used recreational drugs in the past telling me that if they did some drugs while they had cold or flu (I think MDMA or ecstasy, maybe LSD too), the symptoms completely disappeared, but once the drugs are out of their system, the symptoms return. So the infection / virus / whatever was always there, it just wasn’t manifesting in symptoms while the drugs were affecting their immune system.
Just a thought.