noun
person who tested positive
Typically used in medical or public-health contexts for someone who has tested positive for a virus or other infection.
昨日、新たに陽性者が100人確認された。
Yesterday, 100 new positive cases were confirmed.
陽性者は自宅待機が求められる。
Those who test positive are asked to stay at home.
陰性者 is the opposite: a person who has tested negative.
Compound of 陽性 (positive result) + 者 (person).