How does poor hygiene actually help protect against teenage mononucleosis?

Answer 1

In countries with poor sanitation and in cases where young children have poor hygiene, young children can be infected with Epstein-Barr (which causes mononucleosis). These young children have cases that are asymptomatic because a child's cellular immune system is immature and cannot cause severe tissue damage.

With good hygiene, an early childhood infection of Epstein-Barr would be very unlikely. It would then be more likely to show up in the teenage years or later, and cause a more vigorous cellular response, producing the signs and symptoms of mononucleosis in 50% of cases.

Taken from Microbiology- With Diseases By Body System Second Edition, by Robert W. Bauman