Enric Marco, pig333.com
Disease control on farms depends largely on internal biosecurity. One of the most sensitive areas is the post-weaning phase, where, usually, different ages (batches) coincide even though they are separated into different rooms or compartments.
This overlap in ages is what causes certain infections to become endemic, such as PRRS virus infection. In fact, in multi-site systems where the different ages are located in physically separated sites, it is easy for batches to remain negative to infection, and it is not because distance prevents the transmission of diseases by air since there are ventilation systems such as the ones that take in air directly from the outside, that also prevent it.
The difference is that distance prevents the sharing of tools and prevents people from moving from one age to another, thus contaminating them.