Cross-immunity can provide protection against which of the following?

Cross-immunity refers to the phenomenon where immunity to one pathogen can confer some degree of protection against a related pathogen. This occurs because the immune system recognizes similar antigens between the two pathogens, allowing for a memory response to be activated against both. For example, a vaccination against one strain of a virus may offer some immunity against a closely related strain due to shared surface proteins that the immune system can recognize.

Related pathogens often have overlapping characteristics that make cross-immunity possible. This is particularly observed in viral infections, where members of the same family may express similar epitopes. For example, individuals vaccinated against the measles virus may show some protection against related viruses due to the shared antigens.

The other choices do not accurately describe cross-immunity. Only being protected against the same pathogen does not encompass the broader concept of cross-immunity, which specifically involves related pathogens. Unrelated pathogens would not typically trigger the same immune response due to significant differences in their antigens, and stating that cross-immunity only applies to viral infections is too restrictive, as it can also apply to bacteria and other pathogen types. Thus, the correct choice highlights the important role of related pathogens in cross-immunity.