Macrophages are part of the body defences. Those present in the lung are called alveolar macrophages. They ingest and remove invading bacteria and viruses but not in the case of the PRRS virus. Instead, the virus multiplies inside them producing more virus and kills the macrophages. Once it has entered a herd it tends to remain present and active indefinitely.
Some sows remain naive.
This does not however necessarily mean they are not still immune nor does it mean that they have stopped passing on immunity to their offspring. The clinical picture can vary tremendously from one herd to another. As a guide, for every three herds that are exposed to PRRS for the first time one will show no recognisable disease, the second would show mild disease and the third moderate to severe disease. The reasons for this are not clearly understood. However the higher the health status of the herd, the less severe are the disease effects.