Please answer a question about the following article about European Central Bank:

The European Central Bank had stepped up the buying of member nations debt. In response to the crisis of 2010, some proposals have surfaced for a collective European bond issue that would allow the central bank to purchase a European version of US Treasury bills. To make European sovereign debt assets more similar to a US Treasury, a collective guarantee of the member states' solvency would be necessary.[b] But the German government has resisted this proposal, and other analyses indicate that "the sickness of the euro" is due to the linkage between sovereign debt and failing national banking systems. If the European central bank were to deal directly with failing banking systems sovereign debt would not look as leveraged relative to national income in the financially weaker member states.

What is being caused by links between soverign debt and failing national banks?
the sickness of the euro