Final answer:
According to Riis, the conditions in the slums were extremely poor and unsanitary, with crowded and rundown tenements lacking proper sanitation and fire protection. Diseases spread quickly, but Riis's photographs helped bring attention to these conditions and sparked reform efforts.
Step-by-step explanation:
The conditions in the slums, as described by Jacob Riis, were extremely poor and unsanitary. People lived in crowded and rundown tenements, often cramming many families into one apartment. The buildings lacked proper sanitation, fire protection, and indoor plumbing. As a result, diseases such as tuberculosis and pneumonia spread quickly. Riis's photographs exposed the living conditions in the slums and helped generate public support for reform.