Humans are hosts to a remarkable variety of parasites, ranging from single-celled protozoa to multicellular worms visible by the naked eye. Parasitic infections remain a leading global health threat. In 2023 alone, there were over 250 million cases of malaria, causing nearly 700,000 deaths (GBD, 2023). An estimated 2 billion people are infected by helminths (Wright, 2018). While records of parasites date back millennia, the rapid evolution of our understanding of them parallels that of a transformative scientific tool, the microscope. Parasitology has infected nearly all types of microscopy since its invention, illuminating an otherwise invisible biological world.
The naked eye
Long before most parasites were seen, their clinical manifestations were known. Ancient physicians relied on observable symptoms to describe disease. Notably, the Ebers Papyrus, a compilation of Egyptian medical texts from 1550 BCE, is one of the oldest medical records and describes symptoms of parasitic infections. The text mentions bloody urine, which in combination with evidence of enlarged livers, distended stomachs, and calcified eggs in mummies, corroborates that schistosomiasis was endemic in Egypt at the time (Cox, 2002). Larger helminths, like guinea worms, had been observed and attributed to infection (Hoeppli, 2004; Simonetti, 2023). Smaller parasites, especially protozoa, though, remained invisible, causing misunderstood diseases.
Uncovering a smaller world
The invention of the microscope in the 17th century marked a turning point for parasite discovery. Antonie van Leeuwenhoek, the Dutch “father of microbiology,” was among the first people to observe a single-celled parasite directly. Through a single-lens microscope in 1681, he examined a sample of his own stool and described “animalcules,” which we now know was giardia (Steverding, 2025). The use of microscopy in parasitology took off.
In the late 19th century, Charles Louis Alphonse Laveran, looking through a simple light microscope, identified Plasmodium parasites in the blood of malaria patients. This was the first direct evidence that a protozoan caused human disease (Guizetti, 2024). Shortly after, Ronald Ross linked the transmission of malaria to mosquitos. Ross described examining mosquitos that had fed on malaria patients through a “worn out” microscope with a cracked eye piece (Ross, 1902). Together, these discoveries debunked the previously accepted theory that malaria was caused by bad air, or “mal aria.”
By the early 20th century, many parasites had been identified microscopically including Trypanosoma, Leishmania, and Schistosoma. These discoveries established the field of parasitology and initiated early global health campaigns of vector control, sanitation, and targeted treatment.
Parasites in action
Today, microscopy continues to grow our understanding of how parasites thrive. Given that much of a parasite’s life cycle occurs within other organisms, researchers seek to uncover how they interact with their hosts at the cellular and molecular level. This requires imaging techniques that preserve both biological and spatial context.
The more traditional microtomy and histological staining allow for close examination of thin sections of infected tissue. However, advances in confocal and light-sheet microscopy, using optical sectioning, allow for minimally invasive and efficient volumetric imaging, even applicable to live samples. Whole-mount fluorescent microscopy enables the localization of targets by tagging specific proteins, genes, or cellular components. Recent innovations in sample preparation and large-scale imaging have allowed researchers to visualize a whole, schistosome-infected snail, providing insight into infection niches (Poteaux, 2025). Researchers can now not only observe complete structures but also dynamic processes such as invasion, immune evasion, and life-cycle proliferation within hosts.
Parasitizing the parasite
As microscopy continues to evolve, new technology promises even deeper insight into parasite and host biology. Volume electron microscopy (vEM) represents one of the most exciting emerging frontiers for investigating host-parasite interfaces or parasite development, as in this Plasmodium ookinete vEM atlas (Darif, 2025). While there are many different techniques for acquiring vEM datasets, ultimately a three-dimensional reconstruction allows researchers to map entire organelles, cells, or organisms at nanometer resolution to answer complex biological questions.
Like traditional EM, vEM is technically and computationally challenging, requiring expensive equipment, skilled sample preparation, lots of time, and massive amounts of data storage. But, as the field rapidly develops, advances in data collection technology and processing have proven to be a crucial step toward high-throughput discovery.
The cornerstone of diagnostics
Despite advances in molecular diagnostics, light microscopy remains the primary tool for diagnosing parasitic infections. Examining blood smears for malaria or using the Kato-Katz method for detecting helminth eggs in stool provide reliable and cost-effective methods for confirming infection. However, access to microscopy in regions where parasites are highly endemic remains scarce and the slide screening is quite time consuming.
Innovations aimed at lowering the cost of equipment and making them more portable helps to address this gap. One example is Octopi by the Prakash Lab, an automated microscope running on solar or battery power with AI-integrated software that can rapidly detect malaria. Similar systems trained to detect eggs or malaria in samples are now being employed in the field.
Zooming out
From early descriptions of “animalcules” to nanoscale reconstructions of Plasmodium, microscopy has fundamentally shaped our understanding of parasites and their diseases. It has enabled the discovery of parasitic agents, uncovered transmission pathways, and drives diagnostic methods that remain in use today.
As the field of parasitology advances and the need for solutions to pressing global health threats escalates, innovative microscopy has continued to provide avenues for discovery. High-resolution imaging accelerates vaccine and drug development by uncovering parasite and host biology in unprecedented detail, while lower cost tools bring diagnostic capability to underserved populations. Thus, microscopy is not only a tool of discovery but also a driver of equity in global health, bridging the gap between cutting-edge scientific technology and real-world impact and enabling discoveries that push the frontiers of discovery, diagnostics, and global health solutions.