Koch was a German doctor who read about Pasteur's work. With funding, what did Koch decide to do?

• A. Develop a vaccine
• B. Study malaria
• C. Invent the microscope
• D. Investigate whether diseases were linked to specific germs

Answer: (D)

The idea being tested is proving that specific diseases are caused by specific germs. Pasteur had shown that microbes can cause illness, but Koch wanted to identify exactly which germ is responsible for each disease. With funding, he set out to move from this general idea to a concrete method for linking a germ to a disease. He developed a systematic approach—what we now call Koch’s method—to show that a particular microorganism is present in every case of a disease, can be isolated in pure culture, must cause disease when introduced into a healthy host, and can be re-isolated from that host. This made the link between a disease and its specific germ demonstrable rather than just suspected.

The other options miss the point. A vaccine is a therapeutic aim pursued later and by different paths, malaria isn’t the focus of his initial work, and the microscope had already existed well before his time—the breakthrough was in establishing a reliable way to connect a specific germ to a disease.