What organisms do you know that live inside other organisms? Have you felt the influence of such inhabitants on yourself?

The living world in nature is diverse and great. There are organisms that live inside humans, animals, plants. For example, bacteria. These organisms play an important role in the functioning of various systems. In humans, bacteria form the intestinal flora, and they normally aid digestion. But when our body is weakened, these same bacteria can cause disease, they are called opportunistic. There are also some types of worms that live in the body of animals and humans. These are parasites (they feed on their host). They can settle in the intestines, liver, lungs, heart and even the brain (carried by the blood stream). For example, ascaris, pinworm, dwarf tapeworm, bovine tapeworm, cat fluke, etc.