Left parotid infection affects the parotid duct and the tissue of the inside left of the mouth.
In general, infections affect the elasticity of tissue. Cells of infected tissue lose function, and the infected tissue becomes less elastic. The tissue on the left side becomes more rigid than the right. This is seen in both external infections like open wounds and internal infections.
Slightly more rigidity on the left side and slightly more elasticity on the right side result in the left side staying rigid as if hanging in place while the right side extends more easily. The elasticity of the facial tissue on the left and the right is imbalanced, and the facial tissue slants towards the side with more elasticity, and in this case, a right side facial droop occurs.