The tubing isn't attached to the bit that's in you, permanently, Sunny - and neither is the cannula!
You start off with a full reservoir of insulin (the new ones are sterile, so not much chance of letting any pugs in, between unsealing it and sticking the attached needle into the vial of insulin to fill it. Then you fill it with insulin and get rid of any remaining air bubbles (same as filling a syringe) and detach it from the vial. Then you detach the needle and attach the tubing, also sterile until unpacked) and load the reservoir into the pump itself. You tell the pump to prime the tubing, so it fills the tube until insulin starts dripping out of the end.
Then you put the unsealed cannula into wherever you decide to stick it, remove the introducer needle that goes down the cannula tubing in order to get that into you and then attach the 'drippy' end of the tubing to the cannula. Then you tell the pump to prime the cannula, so the insulin fills the cannula tubing itself. The pump can't actually deliver any insulin into you, until you turn it on and tell it to.
And you replace the cannula every two or three days, in a different site depending on how reliable your skin proves to be, at absorbing it which you will discover in use.
Where is infection going to get in, if you've eliminated all the air, and not used contaminated insulin?
