Ethan Snyder felt exhausted last Friday as he took to the streets for a seventh straight night. He figured a week of protesting police brutality and systemic racism alongside thousands of fellow Portlanders would wipe anybody out.

But when his fatigue morphed into a fever, aches and body chills over the next few days, Snyder, 19, decided he should get tested for COVID-19.

He believes the positive result that came Tuesday stems from the many hours he spent marching, chanting and avoiding tear gas, often among throngs of tightly packed strangers.

“It’s the most likely reason why I have it,” said Snyder, a Franklin High School…