The difference in cooking requirements between ground beef and steak is primarily due to the way they are processed and the potential presence of harmful bacteria.

1. **Surface Area**: Ground beef has a much larger surface area than a whole cut of steak. When beef is ground, the process exposes more surface area to potential contaminants, such as bacteria like E. coli or Salmonella, which may be present on the surface of the meat. This increases the risk of foodborne illness if the ground beef is not cooked thoroughly to kill any pathogens.

2. **Internal Contamination**: With steak, any potential bacteria are typically limited to the surface of the meat. When a steak is cooked rare or medium-rare, the high heat applied to the surface is usually sufficient to kill any bacteria present. The interior of the steak remains relatively untouched by contaminants, especially if it is properly handled and cooked under sanitary conditions.

3. **Ground Beef Composition**: Ground beef is made by grinding together various cuts of beef, including trimmings from different parts of the animal. This increases the likelihood of cross-contamination during the grinding process if proper sanitation practices are not followed. Additionally, bacteria present on the surface of the meat can be mixed throughout the ground beef during grinding, making thorough cooking necessary to ensure safety.

4. **Temperature Requirements**: To ensure that ground beef is safe to eat, it needs to be cooked to a higher internal temperature than whole cuts of beef. The USDA recommends cooking ground beef to an internal temperature of at least 160°F (71°C) to kill any harmful bacteria present. This ensures that all parts of the ground beef reach a temperature sufficient to destroy pathogens.

In summary, ground beef needs to be fully cooked to ensure food safety and reduce the risk of foodborne illness due to its larger surface area and potential internal contamination. On the other hand, whole cuts of beef like steak can be cooked to rare or medium-rare doneness because any bacteria present are usually limited to the surface and can be effectively killed by high heat.