Bacteria can affect the human body in which of the following ways?

Bacteria can indeed lead to infectious diseases, which is a major way they impact the human body. Infectious diseases are caused when pathogenic bacteria invade the body, multiply, and trigger an immune response. This can result in a wide range of illnesses, from mild infections like strep throat to severe conditions like pneumonia or sepsis. The effects of these infections can vary greatly depending on the virulence of the bacteria, the route of entry into the body, and the individual's immune response.

The other options, while they can be associated with bacterial presence in some contexts, do not directly stem from bacterial action alone. For instance, while bacteria can contribute to inflammation, inflammation is a broader response and not exclusive to bacterial infections, as it can also arise from viruses or injuries. Cancer and allergies also have more complex etiology and are not primarily linked to bacterial infection in the same direct and systemic manner as infectious diseases are. Thus, the most pertinent and straightforward influence of bacteria on the human body in this context is their role in causing infectious diseases.