<u>Viruses </u>are microorganisms that are not included in phylogenetic trees because they are acellular.
Viruses can be described as the tiniest microorganisms that have the ability to replicate only using the host machinery. Scientists still have a debate on whether to consider viruses as living or non-living entities as they are unable to function as living organisms without a host.
Viruses also are not included in phylogenetic trees because they lack genes that are common to all species. They do not have proper cellular structures and are acellular due to which their addition to any phylogenetic tree is not possible. Any particular gene is not shared through a viral lineage, due to which creating a virus lineage is not possible.
LD50 is a measure of toxicity in animals. A chemical with a smaller LD50 is more toxic and in this question, Shiga toxin has a lower LD50, thus it is more lethal.
