You are of course referring to vaccines. When you give a vaccine the animal or person develops immunity to the virus. The vaccine can contain the actual virus itself and is called a live virus vaccine. It would be similar to taking the actual virus and injecting it (or exposing it) to the animal or person. For example, when people are exposed to chicken pox from another person. In this case you would be exposed to a live virus. This can be very risky as you can cause the animal or person to actually come down with the virus and then have no way of stopping the disease. So they produce a modified virus which was modified so that it is weaker than the live virus. This should then enable to animal or person to be exposed to a weak version of the virus so they would be able to fight the virus (produce antibodies) and thereby develop immunity to the virus without as much risk as in receiving a healthy live virus. A dead virus is just that. The vaccine contains the normal virus but it was killed. This would be very safe in a vaccine since the virus cannot do any harm. The only bad thing is that the vaccine is normally not as strong (in producing antibodies and therefore immunity) as a weaknd (modified) virus. So you normally get a better immunity response to a modified virus vaccine as to a dead virus vaccine. However, I have heard of problems in modified vaccines causing a flare up of the virus so the dead virus vaccine is also a lot safer.