A vaccine uses a live pathogen that is too weak to make you sick. Once it enters your body you defeat it easily. Now that yur body is familiar with it your body can fight off the disease in the future.
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Use warm water, keep the dishes close, and skip the pre-rinse are some ways to save water.
The reasons are:
- the blood is very low on nutrition
- the leech is not feeding itself regularly
The leech slows it metabolism a lot, so it can survive on very low nutrition. The blood is very low on nutrition, and it is the leech's food source, so the evolution made it preserve energy.
The leach doesn't have the opportunity to feed itself on regular basis, so it takes maximum advantage of any opportunity and consumes as much blood as possible so that it can survive long term when it will not have food at disposal.