In 1898 Friedrich Loeffler and Paul Frosch found evidence that the cause of the highly contagious hoof-and-mouth disease in livestock was a microscopic infectious particle. It was discovered that this particle requires a host cell to reproduce. Which pathogen is most likely responsible for causing hoof-and-mouth disease in livestock?

A bacterium, because it is contagious -1
A fungus, because it infects livestock -2
A protist, because it is microscopic -3
A virus, because it requires a host cell to reproduce -4