Fleas carry many diseases and adults can live as long as two month without a blood meal. Fleas are most prevalent during warm and humid months, but are a risk to dogs and cats throughout the year. Fleas will also attack and draw blood meals from humans and can jump as much as 7/8 of an inch vertically and up to 16 inches horizontally, which is where stories of flea circuses are derived.
Fleas range in color from dark brown to black and are very small, measuring from 1/12 to 1/16 of an inch. They have six bristly legs with bristles on their flattened bodies. Female fleas will lay about 25 eggs a day and 800 during their lifespan. They go through a complete metamorphosis during the first two weeks to three months of their lives.
Besides the problems posed by the creature itself, fleas can also act as a vector for disease. For example, fleas transmitted the bubonic plague between rodents and humans by carrying Yersinia pestis bacteria. Murine typhus (endemic typhus) fever, and in some cases Hymenolepiasis (tapeworm) can also be transmitted by fleas. (Factoid Source: wikipedia)