What exactly is a URI?

URI stands for Upper Respiratory Infection.

They usually starts with symptoms like excessive sneezing, runny nose, runny eyes, coughing etc. although they can very quickly develop and become much more serious; resulting in weight loss, loss of appetite, lethargy, extreme difficulty breathing and death.

If cought at an early stage, URIs are easily treated with a course of antibiotics, for example Baytril, on perscription from a vet.

Can they be fatal if caught too late?

Definately.
URIs can take hold very quickly, and very often result in death.

Read this page- Guinea Lynx :: URI and familiarize yourself with the signs. It's imperative that all pig owners know these signs.

Ly&Pigs- Thanks so much for your responses to my posts!

Thanks Cherish! Thanks for letting me know some of the things tolook for!