Subsequent researchers knew enough about bacteria to know how to search for a bacterial pathogen by filtering extracts through special filter paper, etc., but none of those methods worked to find the cause. Someone figured out that it wasn't just toxins produced by an infected plant, but that there was some agent that was reproducing, and that generation after generation, could still infect tobacco plants. Attempts to grow the pathogen on petri dishes were unsuccessful. The smallest viruses are smaller than ribosomes in cells, and the largest are so big that they're just barely visible under the highest power of magnification possible with a regular light microscope. Viruses are obligate intracellular parasites , that is, they can express their genes, do chemical synthesis of more viral nucleic acid and protein, and replicate only within a living cell of the correct host species.
Viruses are capable of replicating many copies of themselves, which then go infect other cells.
However, each type of virus has a limited range of host cells. Herpes viruses are especially known for this, and it is thought that some forms of cancer may be caused in this way. When these replicate, they then burst out of the host bacterium cell, killing it as many viruses are released. To infect a new cell, the membrane surrounding the virus joins with the cell membrane of a host cell, and the capsid and nucleic acid sneak inside. When the new virions leave, each wraps a bit of cell membrane around itself on the way out.