Could womb risk kill dog?

I am unsure whether to have my 18-month-old mixed breed dog spayed. My vet advised that without the operation there is a risk of a womb infection in later life, which can be fatal without emergency treatment. What are the symptoms and how big a risk is it?

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The medical name for this condition is pyometra, meaning pus in the womb. Vets generally abbreviate this in clinical notes to 'pyo'. It is a very common condition in older female dogs that have not been spayed.