As many as 6,000 people may have been infected with the disease.

as 在这里是副词，表示"达到...的程度"或"像...一样"。 many 是形容词或代词，表示"许多的"、"大量的"。 as...as 是一个固定搭配结构，第一个as是副词，第二个as是连词，表示"和...一样多"或"多达"。 as many as 6,000 people 表示"多达6000人"，强调数量之多。