Explanation:

- Melatonin is a hormone mainly known for regulating sleep, but it also has some roles in the immune system.

- It does not directly "encode" proteins, but it can influence the expression of certain proteins in cells.

- These proteins can help defend cells by preventing virus entry.

- Option 1: virus - This is the correct answer. Melatonin helps cells resist viral infection by increasing antiviral defenses.

- Option 2: bacterium - Melatonin has some antibacterial effects, but does not mainly prevent bacterial entry in this way.

- Option 3: fungus - There is little evidence showing melatonin blocks fungal entry specifically.

- Option 4: protozoan - Melatonin does not have a major role in blocking protozoan entry into cells.