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Subject - biology, medicine

Parasites are organisms that live on or in other lifeforms and obtain some or all of their nutrients from the host, often causing damage or disease.

Parasites that live on the surface of their host, such as lice, fleas, or ticks, are called ectoparasites. However, if they settle inside the body, i.e. in deeper bodily cavities, tissues, or blood, they are endoparasites (e.g. tapeworms). Stationary or permanent parasites remain in their host for nearly their entire life cycle; temporary or periodic parasites only live in or on the host for a while and are autonomous the rest of the time. Depending on their nutritional supply needs, parasites are classified as either obligate parasites, which are dependent on their host for their entire lives, or facultative parasites, which can obtain food from both their host and other sources.

Parasites that affect humans include protozoa, worms, and arthropods. In a broader sense, bacteria, viruses, and fungi may also be considered as parasites.

Recommended Learning Units

Cell Cycle, Mitosis, and MeiosisLevel 260 min.

BiochemistryBiological FundamentalsCell Cycle

Cell reproduction is based on the principle of cell division. In order to divide a cell into two identical daughter cells, all of the genetic material and all cell components must be duplicated. Animated illustrations demonstrate the cell cycle responsible for this, as well as illustrating cell division by mitosis and meiosis.