Hi Gaurav,

in principal an "opportunistic pathogen" is a viral, bacterial, fungal or protozoan agent that causes disease only in individuals with a defunct or compromised immune system. In contrast, healthy individuals are in general not affected as their intact immune system is able to cope with the pathogen. In several cases, opportunistic pathogens may well reside within one individual. The gut (Clostridium difficile), skin (Staphylococcus aureus) or nasopharynx (Streptococcus pneumoniae) are prominent habitats for these pathogens. In recent years, this has become increasingly more threatening as many opportunistic pathogens express extensive resistances to antibiotics.

I hope this covers the basis as to what an opportunistic pathogen is.

Best

Sebastian

an opportunistic pathogen, as its name suggests, may be pathogenic when presented with the opportunity. The opportunity may be immune compromise (with many reasons for that), or it may be the microbe finds itself in a site it doesn't usually find itself in: eg a deep puncture wound will drive many of the skin flora deep into the body. Or clinicians may generate the entry portal into people who are already sick (think urinary catheters, or i.v. lines). 

An opportunistic pathogen actually is a pathogenic one if it get some conditions such as increase in number or growing in critical area in the human body and this is only related to the immune system depression, during chemotherapy for example etc.

Opportunists may require a portal of entry and may not be pathogenic without one. That portal could take the form of a break in the physical barrier of the skin or cuticle or epithelial tissues, or it could take the form of an intermediate host or weakened host state.

An opportunistic pathogen is a micro-organism with full potential of virulence that can be expressed as the internal or external conditions are favourable to expression and therfore such opportunistic can led to infectious state. 

these are virulent pathogens that upon invasion of a susceptible plant host, they are capable at causing plant disease and later subjecting the plant to crop failure.

OR

This is a pathogen which is virulent enough to attack a host plant even though the plant is of different pathogen association