
First human challenge study of Covid-19 yields valuable insights about how we get sick
CNN
It takes just a tiny virus-laden droplet -- about the width of a human blood cell -- to infect someone with Covid-19.
That's just one of the findings from research that deliberately infected healthy volunteers with the SARS-CoV-2 virus. The findings were published Thursday in the journal Nature Medicine.
Challenge studies can be controversial because they involve intentionally giving someone a virus or other pathogen in order to study its effects on the human body. Even with safeguards in place, there's an element of risk, particularly when studying a new virus.

The governments of Mexico and the United States signed a memorandum of understanding on Thursday to fund and expedite several wastewater treatment projects in the Tijuana River basin. Untreated wastewater continually affects residents living along the river, which flows across the border from Tijuana and through several of San Diego’s southern neighborhoods.