Campbell has no background in treating infectious viral diseases, or in any kind of scientific research. His entire background has been in nursing. His experience came from nursing in emergency departments (A&E, Accident & Emergency as the British call it). He later earned a PhD in nursing education before becoming a university nursing educator.
In 2008, Campbell began a YouTube channel, providing educational lectures on health science and nursing. Until 2020, his videos received about several thousand views each. With the onset of the Covid-19 pandemic in 2020, however, his YouTube channel began to receive significantly more traffic. He shifted focus from nursing education lectures to major issues related to the pandemic. Much of this increased viewership was among viewers in the US.
In November 2021, Campbell had produced at least two videos that both attracted increased views as well as much criticism for being misinformation. In one video he speculated that wide use of ivermectin in Japan might have been responsible for a decline in Covid-19 cases there. No evidence was provided to support that idea.
And in the other video, Campbell quoted from a non-reviewed journal abstract where the author, Steven Gundry, claimed that mRNA vaccines might cause heart problems. This claim was taken by US anti-vaccers as evidence that Covid-19 vaccinations would lead to a wave of heart attacks among the vaccinated. Again, no evidence ever appeared to support this exaggerated claim.
Because of these debunked claims, John Campbell's videos have been panned as misinformation.
All the above info appeared in a Wikipedia link:
en.wikipedia.org