President Biden has contracted COVID-19 again -- this after several days of testing negative for the virus and seemingly entering the recovery lane, with any illness in the rearview.

JB announced he'd tested positive once more on Saturday, taking to Twitter to write ... "Folks, today I tested positive for COVID again. This happens with a small minority of folks. I’ve got no symptoms but I am going to isolate for the safety of everyone around me. I’m still at work, and will be back on the road soon."

His White House physician, Dr. Kevin O'Connor, supplemented the news with a bit more information and context, publishing a letter that further described Biden's renewed case.

Dr. O'Connor described this as a "rebound" effect, something he says happens with a small percentage of people who catch COVID and are treated with something called PAXLOVID -- which seems to be a medication POTUS was given to help combat the virus and symptoms.

His letter goes on to say Biden tested negative on Tuesday, Wednesday, Thursday and Friday -- but despite this, an antigen test administered on Saturday morning proved to be positive. The doc notes, "The President has experienced no reemergence of symptoms, and continues to feel quite well."

It continues, "This being the case, there is no reason to reinitiate treatment at this time, but we will obviously continue close observation." Even though he won't go on any more meds for COVID, Biden will re-enter isolation procedures out of precaution, just like before.

Earlier this week, he made his official return to work and did a public appearance in the Rose Garden. He'd also told the press he was "feeling great" and was ready to get back to it.