Because such a wide-spread virus would be constantly mutating.
A vaccine for polio is specifically for polio and not for chickenpox. No more than trying to say that a hamburger is a salad.
It can be difficult because when the disease evolves your antibodies will not fit the pathogens and you cannot simply have a different vaccine every week as it would be dangerous to your health.
Live bacteria
An Antonym for vaccine would be Virus
A vaccine is a special version of an antigen the gives you immunity against a disease. A vaccine is a weakened form of disease which helps your immune system create antibodies to fight it off so later if you ever get that certain disease again your body would recognize it and create antibodies to fight against it before it gets too serious.
For the 3-6 months that the vaccine would have provided an improved immunity against influenza, the unvaccinated horse will be more susceptible to contracting influenza than a vaccinated horse.
Some would include.... * Swine flu (although there is one practically ready) * The common cold (although impossible)
Yes, in fact many different vaccines exist for many different diseases (in the case of flu a new vaccine has to be made every year because the virus evolves so rapidly that the previous year's vaccine would not work against it).
You would have to get a vaccine
Just off of common sense, I'm gonna go with a no. It would be difficult to pull out the ring, and make a 'romantic' proposal. Also, you could very easily drop and lose the ring, etc. I would advise you against it.
I would assume that live and weakened vaccine would not be given to such people.
The polio vaccine was tested using human trials to see if it would work and to see if it would cause any problems.