Tetanus - probably the most likely considering that horses do have a tendency to get themselves injured. This disease can be deadly if not seen to quickly and originates from a wound in which a bacteria (clostridium tetani) enters and circulates in the blood stream. This bacteria releases a toxin (I've heard the most powerful known) that basically paralyses the muscles particularly legs, lungs and jaw (hence the common name lock jaw). The tail is also held out rigid. This will eventually cause collapse and death due to complete paralysis of the lungs.
WEE - virus carried by mosquitos. Affects brain and spinal cord. At first symptoms can be quite mild with relatively minor behavioural changes e.g depression and a mild fever. This, however, can progress to more major neuroligical problems such as being off-food, aggressiveness, head-banging, blindness, twitching and facial paralysis. The end is convulsions and death.
Influenza - Finally a relatively mild disease! This disease is rarely fatal amongst adults but mortality is greatly increased in the new-born and young. This shares many of the symptoms of the human form with discharge from eyes and nose, maybe coughing to clear excess phlegm, anorexia, depression and a general stiffness. Care must be taken to give antibiotics to prevent further infection i.e. pneumonia. As it is a virus only symptoms can be treated through good, old TLC! It should clear up in about 3 weeks.
Herpesvirus - This has two types; one that causes respiratory problems, abortion and paralysis (potentially) and another which just causes respiratory problems.
WNV - This is very similar to WEE in that it is caused by a mosquito and it has very similar symptoms; the only difference being a lack of fever.
Rabies - This is another disease which affects the brain. This disease typically has three parts - melancholy, excitement, dumb. IN horses melancholy is apparently quite rare as horses are more likely to become over-exicted, aggressive, sensitive to loud noises and other neuroligical changes. However, once the disease has progressed the horse is quite quickly completely paralysed which results in death.
You can see why we vaccinate...
Well for both the strongyles are what could definitely be classed as *nasty* although I would argue that all worms have the potential to be deadly whether the worms are young or old. Saying that though the round worms in the young. I would take a good guess at ivermectin as everything which contains it says 'we are the best'.
Thanks for the good luck! I'm off tomorrow morning to be in Cam. for the interview on monday so thank you very much Ryle for your questions.