Donald Trump’s quick recovery from Covid-19 last month, advertisements appeared on the dark web — the seamy underbelly of the internet — selling what they claimed was Regeneron Pharmaceuticals Inc.’s drug that the president touted as a ‘miracle’ cure – even though it hadn’t yet been approved for public sale.In May, with coronavirus cases surging around the world and no end in sight, a supposed Spanish laboratory doctor promised Covid-19 infected blood and sputum.