I had a five month old Jack Russel mix get the Parvo virus back in 2002. He was colored like a German Shepard and at first glance the vet thought he was a Shepard mix and said the pup was much too young to survive if he was only a few weeks old. I told him he was a few months old and he was not a Shepard mix, but a Jack Russell mix pup. The vet said, survival chance was much better on a dog the older he gets and most don't get it past 2 years old. Three days at the vet clinic with IVs did the trick. My fury friend lived to be thirteen years old. I learned my lesson. Those puppy shots are important.
The immune system would likely be stronger with age, so this could be the answer.