Foot-and-Mouth disease is primarily a disease that affects cloven-hoofed animals and is caused by the foot-and-mouth disease virus (FMDV). Let`s look at the options:

Option 1: Bacterium - This is incorrect. Bacteria are single-celled microorganisms and while they can cause many diseases, Foot-and-Mouth is not one of them as it`s caused by a virus, not a bacterium.

Option 2: Fungus - This too is inaccurate. Fungi are a group of spore-producing organisms feeding on organic matter, and while they can cause diseases like ringworm or athlete`s foot, they are not the cause of Foot-and-Mouth disease.

Option 3: Protozoan - Again, this answer is incorrect. Protozoans are a diverse group of unicellular eukaryotic organisms, historically placed in the Kingdom Protozoa. They can cause diseases such as malaria or giardia, but not Foot-and-mouth disease.

Option 4: Virus - Yes, this is the correct answer. The Foot-and-Mouth disease is caused by a virus which is highly contagious and affects animals like cattle, pigs, sheep and goats. This virus is responsible for significant economic loss in livestock industries worldwide