Spring and Fall shots given together????

Most vaccines are killed or "modified live" virus vaccines (I say most because some of the vaccines we give in horses are bacterial vaccines instead of viral) --these vaccines CANNOT cause disease. The only vaccine currently in use in horses that can possiby cause disease is Strangles vaccine and that is a risk based vaccine and is not the actual bacteria that causes Strangles naturally but a similar bacteria which can cause symptoms similar to Strangles or can cause a bacterial infection and abcess at the site of injections (or wounds) given at the same time as the strangles vaccine.

The reason you want to vaccinate only healthy horses is because the vaccine isn't what protects them from future infection but the reaction of their immune system to the vaccine. In sick horses, they don't respond as well to the vaccine thus don't build as good (or even any) immunity to whatever you are vaccinating against. Also if you vaccinate a sick horse their body thinks that there is something else attacking it and tries to respond to this new "invasion" thus taking resources away from fighting whatever actual disease/infection is already causing problems in the body.