I live in Canada in the middle of the fentanyl crisis, and I get 90%+ China White that is basically pure Diacetylmorphine made by chemists in a lab, and 90%+ Afghani brown/beige sugar. I never buy IRL in person from dealers, all that stuff is cut and laced and straight junk with fentanyl and benzos. The deep web/darkweb is a gift.
How do you figure it extends the duration of illness? There's no way to measure or corroborate that in any way. Medically speaking, this idea does not hold up to scrutiny, as marijuana doesn't have an impact on your immune system in that manner. Not every bug is the same. There's no way to judge that. And even if every bug were the same, the state of your body is constantly changing in real ways that do affect your immune system. Time of infection, method of infection, what your body experiences from external sources while becoming or dealing with an infection in early stages is proportionate to duration of illness. Marijuana is, to your immune system, almost wholly benign.
Pie
Most certainly it would not be a fun time at all. Psychedelics let you experience more of what you are experiencing. Marijuana is a dissociative, and the dissociation (and to a lesser extent, stimulation) is what's responsible for its aiding in illness relief. That is why dissociative substances are sometimes included in flu/cold OTC/Rx medications.