Interferons curb infection of

The correct option is None of the above.

Interferons (IFNs) are a group of signaling proteins (cytokines) made and released by host cells in response to the presence of pathogens, most notably viruses. When a cell is infected by a virus, it releases interferons to signal neighboring cells to activate their antiviral defense mechanisms, thereby limiting the spread of the viral infection.

Let us evaluate the provided options:
1. Cancer: While interferons have immunomodulatory properties and are sometimes used in immunotherapy to treat certain cancers, cancer is a disorder of uncontrolled cell division, not an external infectious pathogen.
2. Fungi: Fungal infections are primarily targeted by other components of the immune system (such as neutrophils and macrophages) and are treated with antifungal medications rather than being curbed directly by interferons.
3. Bacteria: Bacterial infections are chiefly controlled by antibacterial immune responses (like phagocytosis and antibodies) and treated with antibiotics. While some interferons can help activate immune cells to fight certain intracellular bacteria, they are not the primary line of defense against bacterial infections.

Because the primary target of interferons is viral infections, and "Viruses" is not listed as an option, the correct answer is None of the above.