Willis-Karp's advice is likely the most sound. To paraphrase: don't challenge immunity with alcohol or sugar (and I might add, don't smoke ANYTHING as even "harmless" marijuana inflames lungs and increases risk of severe disease... as well as wrecking REM sleep, and from my own anecdotal experience, virtually stopping remembered dreaming entirely... cannot be good!)
The reference to the beach and sand having beneficial effects is a reference to decades old weak research linking exposure to soil bacteria with reduced depression and improved mood. Willis-Karp's advice to take probiotics makes much more sense as garbage left on the beach is more likely to make microbial exposure be to organisms such as aspergillus flavus (common mildew which produces aflotoxin beta, a serious toxin and carcinogen) and aspergillus niger (black toxic mold).
HIV didn't come from "Polio vaccine from Africa". If someone could go ahead and run a viral comparison study, the not-entirely-impossible assertion that HIV 1 is the decendant of the SV-40 which was found to have contaminated the original Salk polio vaccine, could be either confirmed or debunked.
I myself would extend advice of Willis-Karp in the article to simply more generally : not stress immunity, chemically nor behaviorally; enhance immunity in practical ways, provided you aren't in evident immediate danger of provoking a cytokine storm; oh yes! And have fun and do happy things!