Traders and travellers carried disease-causing germs along with goods across long distances. When European sailors arrived in America, they unknowingly brought germs like smallpox. America's original inhabitants had no immunity against these diseases, so smallpox spread rapidly, killing entire communities and paving the way for European conquest.
The examiner expects you to link trade/travel directly to disease spread with a specific example. Mention of smallpox in America (no immunity → decimated populations) is the key point from the chapter. Two marks = one clear mechanism + one example. Avoid writing about Rinderpest here as that relates to colonialism/cattle, not trade spreading disease.