I see some Trump supporters are saying that Trump's latest comments pertain to blood irradiation therapy / Intravenous Light Therapy, and that those mocking him are either arguing in bad faith or have failed to understand the obvious.

However, the context is more readily explained as Trump extrapolating from the effects of disinfectants and UV light on surfaces. Indeed, Trump appears to suggest it is his own idea.

One Trump supporter has been RTing a partial screenshot of the Wikipedia page for blood irradiation therapy, although he's cropped out the sentence "This issue is subject to skepticism" following claims about benefit to the immune system. https://en.wikipedia.org/wiki/Blood_irradiation_therapy

In 2016 @hwallop had a dose of light injection for a consumer piece in The Times. Doesn't seem to have been particularly impressed. But this sort of thing is presented as a booster, not as a cure for infections. https://www.thetimes.co.uk/article/could-a-vitamin-drip-really-reboot-my-tired-brain-xrgnwwzl0

Injectable light!

There is also extracorporeal photopheresis, where the blood is removed and treated with ultraviolet. It has a legitimate use with some diseases, such as lymphoma https://www.guysandstthomas.nhs.uk/our-services/dermatology/specialties/extracorporeal-photopheresis/overview.aspx

In 2004 there was a case where a teenager nearly died when the external procedure was incompetently applied by a naturopathic medical practitioner. It was reported in the Rocky Mountain News

In 1996 a certain Robert Jay Rowen MD published "Ultraviolet blood irradiation therapy (Photo-Oxidation): The cure that time forgot" in a journal called the International Journal of Biosocial and Medical Research. He made bold claims for its efficacy. http://drferchoff.com/files/ubiarticle.pdf

The intravenous version is popular in Russia, where it known as ВЛОК (Внутривенное лазерное облучение крови) https://aif.ru/boostbook/vlok.html