Update ncov_trees-background_it.md
@@ -60,6 +60,7 @@ Quando il patogeno si replica e si diffonde, il suo genome ha bisogno di essere
<img width="500px" alt="cartoon showing how transmission tree and phylogenetic tree relate" src="https://github.com/nextstrain/nextstrain.org/raw/master/static-site/content/help/01-general/figures/infection_tree_combined.png"/>
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The illustration above shows a sketch of a transmission tree. Each circle represents a case (infected person), with horizontal lines indicating the duration of their infection. Connected cases represent transmissions from one person to the next.
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Here, we see the full picture of the transmission tree. In practice, however, only a subset of cases are sampled (blue); the transmission tree is unknown and typically only rough estimates of case counts are available. Genome sequences allow us to infer parts of the transmission tree. In this example, three mutations (little diamonds) are indicated on the tree. Sequences that have the same mutations are more closely related, so these mutations allow us to group samples into clusters of closely related viruses that belong to the same transmission chains.
