Microsporidia are a group of microscopic parasites that spend part of their lives inside the cells of a broad range of animal hosts, including humans. These parasites are considered to be related to fungi, some of which also live within the cells of other species and are known as fungal endoparasites. One of the shared characteristics of these parasites is that they cannot make nucleotides, molecules that are both the main source of energy of the cell and also the building blocks of DNA. Instead, they take nucleotides, or the materials needed to make nucleotides, from their host cells. Once Microsporidia have depleted a host cell, they turn into spores that can survive outside the host until they invade a new cell, starting the cycle anew.
Major, P., Sendra, K., Dean, P., Williams, T. A., Watson, A. K., Thwaites, D. T., Embley, T. M., & Hirt, R. P. (2019). Stealing is the Deal: Parasites that grow within animal cells use a family of transport proteins to take nucleotides, the building blocks of DNA, from their hosts. eLife. https://elifesciences.org/digests/47037/stealing-is-the-deal