- Viruses contain nucleic acids surrounded by protein coats and must infect host cells to reproduce, while cells have membranes, organelles, and can carry out life functions independently. - Bacteria play both beneficial roles like digestion and harmful roles causing diseases like strep throat and tuberculosis. - Mutualistic relationships benefit both organisms, like clownfish and sea anemones, while commensalism benefits one without affecting the other, like egrets near rhinos.