Cats are extremely territorial animals. Most of the fights break out when territory is contested. Once territory is established, they no longer fight. Cats doing small excursions to other's territory leave quickly if enemy is sighted. For example, my cat has territorial dispute with one other cat over next garden. Our garden is obviously my cat's so a fight never breaks out in our garden, i.e., challenger leaves our garden if he sees my cat. However, next garden is still contested and I wake up to screams now and then.
Cat fights, as long as medical treatment is promptly given, are not that dangerous. Domesticated cats are better fed and generally stronger than stray cats. Therefore, there is some chance the stray cat will never pick a fight and just growl.
- Angled fences are great ways to prevent cats from getting into your garden. Make sure that spacing is narrow enough to prevent the cat from strangling himself by getting caught to the fence.
- Cats hate smell of citrus. You can use it on the stray cat's point of approach.
- Do you let your cat to defecate in the garden? One way for a cat to show dominance is to defecate without burying his excrement. Smell from fresh excrement deters other cats. Note that cats try to pick fights that they can win, and unburied feces scream confidence :)
- Do you trim your cat's nails? Cats feel unsafe when their nails are trimmed. It is generally not a good idea to trim the nails unless he is solely an indoors cat.
Specific to your concerns, cats have low susceptibility to tetanus, meaning while they can get tetanus, it happens rarely, so I wouldn't worry that much about tetanus. Cats might also lose an eye in a fight but prompt medical care reduces this chance.
If I were you, I would not worry much about this. If you can't erect a fence or can't prevent the other cat by citric smells, have a few boxes with small openings around the garden. They would help either the hostile cat or your cat to retreat to if the fight goes real and you'll be alerted by their screams. Once there is a barrier between them, fights become more paw to paw action, which is generally safer for them as their fur protects them from easy cuts and scrapes that would be nasty to a human.
One final comment, most cats start chasing if the other cat retreats in a hurry. So, when they are close and scream at each other, if you come hurried and scare the hostile cat, your cat will start to chase it. In that case, the fight might occur in an area where you can't reach easily for medical treatment or where the hostile cat feels more at home and therefore becomes more aggressive. If they are screaming at each other, wait a little until they resolve the dispute cat to cat.
