Final answer:
North America is the continent that lies entirely in the northern and western hemispheres and is the third-largest continent in land size. Sub-Saharan Africa is the region with the majority of HIV-positive cases and was most directly involved in the Trans-Atlantic slave trade.
Step-by-step explanation:
The continent that lies completely in the northern and western hemisphere and is the third largest in land size is North America. It fits the geographical description and ranks behind Asia and Africa in terms of physical area. When looking at the issue of HIV-positive cases, the continent home to the overwhelming majority of these cases is Africa, specifically sub-Saharan Africa.
Africa is noted for its diverse geography, which ranges from the vast Sahara Desert to tropical rainforests. It is the second-largest continent with a significant number of independent countries, each with its own unique languages and cultures. During the fifteenth century, European countries began establishing trading ports along the African coasts, marking the beginning of significant foreign influence in the region.