According to business insider, the New York University microbiologist Philip Tierno claims that bedsheets should be washed once a week and that mattresses should be changed more frequently because otherwise it can "make us sick" and trigger allergic responses due to inhalation of fungi etc.
But can microorganisms and fungi really flourish in bedsheets with only sweat as their feed? And if they were to flourish to dangerously large colonies, wouldn't they be visible to the naked eye (spots with a slight, or major, color change)? And even if they are inhaled etc., is there actually a link to health? It appears that while bedding may contain fungi, the concentration threshold for imposing a health hazard is unknown, nor if such a threshold is reached in the case of bedding.