A new study of a small group of workers at industrial hog farms in North Carolina has found that they continued to carry antibiotic-resistant bacteria over several days, raising new questions for public health officials struggling to contain the spread of such pathogens.

Although the bacterium, Staphylococcus aureus, is common and does not always cause illness, it can contaminate food and give rise to skin infections and respiratory diseases. Its methicillin-resistant variation, known as MRSA, has wreaked havoc on hospital systems, causing life-threatening complications.

The study focused on hog farms because previous research had found the highest incidence of S. aureus among workers in those settings.

As of 2012, the most recent year for which data is available, there were an estimated 75,309 serious infections from MRSA and an estimated 9,670 deaths, according to the Centers for Disease Control and Prevention.