[anonymous]

Excerpt from: https://www.petcarerx.com/article/what-diseases-do-fleas-cause/163
Fleas may be extremely small, but dangerous things sometimes come in small packages. Guard your dog against the many diseases that fleas carry and transmit.
You may think that these tiny pests only make your pet do a little scratching and itching, but fleas actually cause a number of diseases, some of which can be fatal. By not protecting your pets from fleas, you are exposing them and yourself to severe health risks. Here are some of the diseases your pet could contract:
Plague
Yes, this is the same plague that caused the death of over a third of the European population back in the Middle Ages. While it’s no longer common for humans to get the plague, your pet can still become infected. A flea that has taken a bite out of a infected wild animal, like a prairie dog, chipmunk, or vole, can easily infect a dog or cat and pass along the disease.

Plague-infected cats and dogs will have a fever, swollen lymph nodes, and may even die suddenly without much warning. If you think your pet has the plague, you need to contact your veterinarian immediately and isolate your pet from other animals as to stop the plague from spreading.
Cat Scratch Disease
While Cat Scratch Disease usually does not affect cats in a negative way, it puts their owners at risk. Fleas transmit CSD from one cat to another, and close to 40% of cats will carry this disease at some time in their lives. Humans get CSD when infected flea feces on a cat’s claws or fur is transmitted from the pet to their owner through a bite, lick, or scratch. CSD can cause fever, headaches, and fatigue in humans, as well as make those with a weakened immune system seriously ill.
Tapeworms
How does a dog biting a flea end up with your pet carrying tapeworms? Simple. If the dog eats a flea that is carrying tapeworm eggs inside their bodies, then that pet could soon become infected. Once inside your dog, the tapeworm hatches and attaches itself to your pet’s intestines, causing weight loss, vomiting, and irritation.
Flea Allergy Dermatitis
All it takes is one bite from a flea to cause Flea Allergy Dermatitis, leading to hot spots and extreme itchiness for your pet. Dog hot spots are infected patches of circular skin that could soon become a big wound if your pet continues to itch and bite at it. Even your dog’s tongue licking the wound spreads the bacteria around. If your dog has a hot spot, you should take them to see your vet as soon as possible.
Haemobartonellosis
While usually transmitted by ticks, haemobartonellosis can also be carried by fleas. The disease targets red blood cells and can range from mild to very severe symptoms. If very severe, haemobartonellosis can cause cats to suffer anemia that results in weight loss and a fast heart rate. In some cases, infected cats have been observed eating dirt, and without treatment, cats can die from this disease.

Haemobartonellosis is most likely to affect dogs who have had their spleens removed. It can cause loss of appetite and weight loss for our canine friends.

What’s the best way to keep your pet from suffering from any of these afflictions? Treating them for fleas, of course! There are many options you can choose, from spot on solutions to oral pills. Decide which one is best for your pet, and keep them healthy year round!