Viruses are too small to be seen by the naked eye. They can't multiply on their own, so they have to invade a 'host' cell and take over its machinery in order to make more virus particles. Virus infects all of the given organisms. Mycoviruses are viruses that infect fungi. Many different viruses can infect plants - certain crops are well known to be affected by virus diseases including geraniums, roses, Easter lilies, dahlias, gladiolus, and tulips. Bacteriophages (like Coliphage T4) infect bacteria. All three are correct.