Sentence:  The concentration and apparent localization of the WTV antigen in pseudophloem tissue of the tumor may indicate that the <WSD>virus</WSD> preferentially multiplies in the phloem and may need to be directly placed in this tissue in order to infect plants . 
Meaning of the word marked with <WSD> tags:  (virology) ultramicroscopic infectious agent that replicates itself only within cells of living hosts; many are pathogenic; a piece of nucleic acid (DNA or RNA) wrapped in a thin coat of protein