YES!!!
New etiologic agents are regularly added to the list of infectious diseases, particularly with the availability of nucleic acid amplification techniques for detecting and identfying otherwise non-cultivable micro-organisms.
Several new factors must be taken into account:
Global travel
Lifestyles
Cyclical nature of disease trends
Sexual experimentation
Enviornment
Ethnicity
Recreational drugs
Over-usage of antibiotics
For a list of new diseases discovered in the last 20 years:
here
and also here