microparasite
英[maɪkrəʊ'pærəsaɪt]美[ˌmaɪkroʊ'pærəˌsaɪt]
A microparasite is a parasite that completes a full life cycle within one host and can be transmitted directly to conspecific hosts. They often reproduce within a host's cells and are generally too small to be seen with the naked eye.