Complex sentence: Human infection is caused by about 21 of 30 species that infect mammals: the different species are morphologically indistinguishable, but they can be differentiated by isoenzyme analysis, DNA sequence analysis, or monoclonal antibodies.
Split into simpler sentences: Human infection is caused by about 21 of 30 species that infect mammals. The different species are morphologically indistinguishable, but they can be differentiated by isoenzyme analysis, DNA sequence analysis, or monoclonal antibodies.