The annual flu season begins when the sun’s intensity decreases. During the flu season we have less light than in the spring and summer.

In 1981, Edgar Hope-Simpson proposed that sunlight explained why the flu was seasonal. UVB light from the summer sun triggers robust vitamin D production in the skin if you spend enough time in the noonday sun without sunscreen.

A natural steroid hormone, the vitamin D produced in your body has profound effects on human immunity. For one, it dramatically stimulates potent antimicrobial peptides in the cells lining the respiratory tract which play a major role in protecting the lungs from infection.

Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2870528/

A regular boost of UVB, either from sources like the Blu Room or from summer sunlight, reduces the incidence of respiratory infections, aka “the flu.”