From a scientific paper of Max-Planck-Institute quoted:

The history of virus research in the 20th century is usually portrayed as a continuous process, as a history of progressive revelation of virus nature (see Waterson 1978: xii; Hughes 1977: 75 ff.; for a critique of this concept see van Helvoort 1994a:187).

However, our analysis of the case study material has revealed some things that lead away from such a view of history. In particular, it was shown that the refinement and expansion of experimental means and procedures, which is generally seen as a guarantee for uninterrupted progress in the knowledge of nature, had rather led to setbacks in the period under consideration (for example, in the development of virus classification) and had deepened the gap between the contending parties in virus research.

With the "filterable" virus, something had been discovered of which, according to the concepts handed down, which had after all mostly proven themselves in the research of infectious diseases, no picture could be formed which all researchers could have shared.

Very different interpretations of the nature of this phenomenon emerged and were put forward against each other. No experimental proof for this or that concept, which all researchers would have had to recognise, could be presented by either side.

This means that the decision as to whether this or that explanation most accurately expresses the "true" nature of the virus could not be "objectified" empirically.

Page 57 ff recommended reading (language: German). Source: https://www.mpiwg-berlin.mpg.de/Preprints/P125.PDF

Also: read my post where I extracted it from: https://hive.blog/imformationwar/@erh.germany/what-fills-the-gap-if-you-don-t-believe-in-the-contagion-theory