By: Doris Calloway DVM
You have to bring Fido to see the vet for his diarrhea.  He’s due for his vaccines, has not been vaccinated yet and you think he may have Parvo, can’t you get his Parvo vaccine now to help him? Unfortunately, the answer is no.  
Vaccines are great – they protect against horrible diseases that there are no cures for.  They work by telling the immune system to make antibodies against those diseases. This is great when your pet is healthy, but not when he’s sick.   When your pet is sick, vaccines basically act like a distraction – they pull your immune system’s attention away from focusing on why your pet is sick. 
If Fido is having diarrhea (or has an ear infection, or is coughing, etc), we need his immune system to fight whatever’s wrong with all its might.  One hundred percent of his immune system needs to be fighting whatever is causing his diarrhea.  We don’t want to have his immune system giving 50% of it’s attention to make antibodies for the vaccine.  
If we give vaccines while something else is going on, he could potentially get sicker.  It’s just not worth the risk to vaccinate while your pet is sick.  It’s much safer to wait until Fido’s recheck and have the vaccines done at that time (assuming Fido is all better!).