The New York State attorney general has issued a cease-and-desist order to Alex Jones, the conservative radio host, alarmed by false claims on his website that his diet supplements and toothpaste could be used to fight the coronavirus.

Mr. Jones, according to the attorney general, made a series of claims: That his products could act as a “stopgate” against the virus, that his Superblue brand of toothpaste “kills the whole SARS-corona family at point-blank range.”

There are no products, vaccines or drugs approved to treat or cure the virus.

As the disease spreads across the United States, so too has online misinformation and the marketing of fraudulent products that claim to prevent the coronavirus, presenting government officials with a new frontier in their escalating fight against the outbreak.

Sham products, from dietary supplements and food to medical devices and purported vaccines, have popped up on social media and digital marketplaces. Masks and respirators that were counterfeit or deceptively labeled have been listed on Amazon and eBay.