Specific-pathogen-free (SPF) is a term used for laboratory animals that are guaranteed free of particular pathogens.
For example, absence of respiratory pathogens such as influenza is desirable when investigating a drug's effect on lung function.
The animals can be born through a caesarian section then special care taken so the newborn does not acquire infections, such as use of sterile isolation units with a positive pressure differential to keep all outside air and pathogens from entering.
This is because the animals have no protective bacterial microbiota on the skin or in the intestine or respiratory tract, and because they have no natural immunity to common infections as they have never been exposed to them.
Mice raised under SPF conditions (no Helicobacter pylori) were shown to develop colitis rather than enterocolitis.