Prevention Is Key
Of the varying age groups, young puppies are at the greatest risk for dying from a disease, followed by older puppies, seniors, and geriatrics. Also at an increased risk are dogs of any age that are already in a weakened state from illness or suffering from an immune system that is not functioning correctly (immunocompromised). Even robust, healthy adults are not without some risk for becoming seriously ill.
In addition to your Chi's age and general health, other factors that affect how at risk your Chihuahua is include the area of the country you live in; what type of housing you own (such as a farm with livestock or a city apartment); how much and where you travel with your dog; how often you board your dog; and the number of dogs your Chihuahua regularly comes in contact with (as at a dog park that dozens of dogs use daily or dog shows with hundreds of dogs).
Many deadly diseases are regional in nature. If you travel a lot with your dog, it's important to research what diseases are common in the area to which you are traveling. Protecting your Chihuahua may be as simple as making sure she only drinks bottled water, or it may require that your Chi receive a specialized vaccine prior to traveling.
You can't, of course, keep your Chihuahua in a protective bubble all her life. Even if you are meticulous about reducing the chances that your Chihuahua will come in contact with disease, it's not a matter of if your Chi will be exposed but when. Simply said, it's unavoidable. What you can do, however, is provide her with as much protection from contagious diseases, bacterial infections, and parasites as possible through preventive veterinary care.