Fleas can cause allergic reactions and carry tapeworms. Ticks can carry Rocky Mountain Spotted Fever, Lyme disease, and erlichiosis.
Mosquitoes can spread heartworm.
Prevention takes just a bit of care. If Fido spends lots of time in woods or tall grass or weeds, check him over for fleas and ticks every day or two. Remove these critters with a fine-toothed comb and drop them in a vial of alcohol. Reduce mosquito breeding pools by eliminating standing water in the yard. Check with your veterinarian for information about the new, less toxic flea, tick, and heartworm preventives.