Common risk factors can create statistical associations. This work confirmed that ignoring them would lead to wrong conclusions. Ignoring them would indeed result in an over-estimation of the number of interactions as any association, biological or statistical, would be put in one basket. The loss of significance after controlling for other factors was illustrated in this paper with feline viruses data, and was previously found by Behnke et al. [39] for helminth parasites of the wood mice. The next step was to identify an appropriate way to take those risk factors into account.