Figure 1. The share of wealth of the richest 10% in Europe, 1300-2010
Source: G. Alfani, "Economic inequality in preindustrial times: Europe and beyond", Journal of Economic Literature, 59(1), 2021, pp. 3-44
Throughout the seven centuries covered in Figure 1, the Black Death triggered one of just two phases of systematic inequality decline (the other being connected to the two World Wars and the difficult interwar period). In the aftermath of the plague, the richest 10% of the population lost hold of 15%-20% of overall wealth. This decline in inequality was long-lasting: it would take 3 centuries before the richest 10% could regain control of overall wealth to the same extent they had in the pre-plague period. As is discussed in more detail here, there were two main causes behind this reduction in inequality. The first one was the aforementioned increase in real wages of skilled and unskilled workers, and the general improvement of labor conditions, resulting from the greater bargaining power enjoyed by the workforce. Secondly, there was a fragmentation of the large patrimonies, caused by extremely high mortality in the context of the partible inheritance system, which in the late Middle Ages characterized many European areas, like Italy. This resulted in many people inheriting more properties than they needed or wanted. Consequently, it led to an unusual abundance of property being offered on the market. Together with higher real wages due to labor scarcity, this helped a larger part of the population gain access to property.
It would be incorrect to infer from the case of the Black Death that all major pandemics have had the same effects. The reduction of inequality that followed this horrific episode was specifically due to two factors: the extremely high mortality rate, and the pre-plague institutional framework. This framework was not set up to face the unprecedented situation introduced by the pandemic and to counteract its undesired (by the economic and social-political elite) "equalizing" effects.