I assume you are referring to Schengen, which is an area within the EU. Borders have always existed even within the Schengen area and have been open and closed on various occasions during the last decades. Having an open border makes zero difference to the transmission of a virus. Millions of people travel everyday in Europe, many in the Schengen zone. If there were border checks between Schengen zone countries, they would be checking an id card or passport at the border control, in the same way as in any other part of the world. The viruses don't need any of these documents and would freely travel on the bodies of these people anyway. So, it's a fallacy that a border will stop the transmission of such an infection. The UK for example, isn't part of the Schengen zone and has full border control at its airports and ports. Yet it has one of the highest infection rates in Europe. Further afield is the US with its strict border controls lauded by Trump and yet it will probably end up having the largest infection rates outside of China. The excuse that borders are symptomatic to spread of disease is normally one spread by xenophobes for their own agendas.