Let’s try something a little different in this high-yield review post.
Instead of long paragraphs and walls of text, let us provide you with only the high-yield facts on parasites and fungi in chart format—because let’s be honest, when you’re studying microbiology, sifting through endless textbooks can feel like a slog. Plus, you’re likely already reading in depth material with other texts and questions that you have to read through.
Below you’ll find charts of the common parasitic and fungal entities. Each chart breaks down the key organisms, clinical features and high-yield associations. Remember, this is the abridged version and is far from being comprehensive. Instead, think of it as your rapid-fire review—just enough to jog your memory before an exam or clinical rotation.
We’re not going deep here—that’s on you. If you want more detailed explanations, definitely go read those big ID textbooks or dig into Uptodate. But if you want the quick hits and clutch facts you actually need to know? This is it.
Scroll down, save the charts, and get ready to crush the bugs.
High-Yield Microbiology Topics for USMLE Step 1: Parasites & Fungal Infections
Parasites
Parasitic infections remain a global health challenge, particularly in resource-limited and endemic settings. Parasites are increasingly relevant due to travel and migration. These infections are caused by various organisms and can range from asymptomatic carriage to life-threatening disease.
Common Culprits
Common culprits include the single-celled microscopic protozoa, which multiply within the host, often causing acute or chronic infections.
Examples include Giardia lamblia, Plasmodium spp. (malaria), Entamoeba histolytica, which typically spread via fecal-oral route, vectors, or contaminated water. The other main category of parasitic infections are helminths, multicellular parasitic worms, which can cause disease through mechanical damage, nutrient deficiency, or immune responses.
Clinical Presentation
Clinical presentation depends on the organism and host factors, but often includes gastrointestinal symptoms, anemia, or systemic manifestations like fever and eosinophilia.
Diagnosis & Treatment
Diagnosis may require stool exams, serology, or imaging.
Treatment varies—metronidazole, albendazole, ivermectin, and antimalarials are frequently used.
When facing parasites, you MUST ask about travel history and exposure risks during patient encounters—it could make all the difference in diagnosis and timely treatment.
1. Protozoa (Single-celled parasites)
| Organism | Disease | Key Features | Clinical Pearls |
| Entamoeba histolytica | Amebiasis (bloody diarrhea, dysentery and colitis, liver abscess) | Stool O&P, enzyme immunoassay to diagnose, treat with metronidazole | Can cause liver abscess. Transmitted via cysts in contaminated water (fecal-oral) |
| Giardia lamblia | Giardiasis (watery, foul-smelling diarrhea, nausea, bloating, gas and dehydration) | Stool O&P or ELISA, Trophozoites with “face-like” appearance.Treat with metronidazole | Common in hikers who drink contaminated water |
| Cryptosporidium spp. | Watery diarrhea, fever | Acid-fast oocysts. Stool O&P | Waterborne. Major cause of chronic diarrhea in HIV patients. |
| Plasmodium spp. (falciparum, vivax, ovale, malariae) | Malaria – cyclical fever, systemic symptoms including headache, aches, nausea | P. falciparum (most severe) → cerebral malaria. P. vivax/ovale → dormant liver hypnozoitesDiagnose with Blood smear | Antimalarail drugs Atovaquone-proguanil, Doxycycline, mefloquine, hydroxychloroquine |
| Toxoplasma gondii | Toxoplasmosis (headache, confusion, fever, seizures) | Brain abscesses in HIV (CD4<100); chorioretinitis, hydrocephalus in neonates+Toxo IgG | From undercooked meat or cat feces.Ring enhancing lesions |
| Trypanosoma cruzi | Chagas disease | Dilated cardiomyopathy, ventricular arrhythmias, toxic megacolon | Seen in Latin America; “kissing bug” vector. |
| Leishmania donovani | Visceral leishmaniasis (kala-azar) | Spiking fevers, hepatosplenomegaly, pancytopenia | Sandfly transmission. Macrophages filled with amastigotes. |
| Naegleria fowleri | Primary amoebic meningoencephalitis | Acute fever, confusion, photophobia, Rapidly fatal. CSF with trophozoites | Found in warm freshwater. Enters via cribriform plate. Think swimming + meningitis. |
| Trichomonas vaginalis | Vaginitis | Green, smelly discharge, vaginal inflammation and itching, pH > 4.5 | Motile trophozoites on wet mountTreat both patient and their partners with flagyl |
2. Helminths (Worms)
Nematodes (Roundworms)
| Organism | Disease | Key Features | Clinical Pearls |
| Ascaris lumbricoides | Ascarisis (intestinal obstruction, Loeffler’s pneumonia) | Eggs are ingested and hatched into larvae, leads to liver, lung and intestinal issues | Diagnose with ova in stool. Treat with -azoles such as albendazole. |
| Enterobius vermicularis | Pinworm (anal pruritus) | Scotch tape test – see eggs | Common in children, causes itching at night. Treat patient and household contacts with albendazole. |
| Strongyloides stercoralis | Strongyloidiasis (abdominal pain, bloating, diarrhea, skin rash) | Autoinfection leads to persistent infection and eosinophilia | Look for eosinophilia and larvae in stool. Treat with albendazole or ivermectin. |
| Ancylostoma | Hookworms → anemia | Worms leave itchy red/brown tracts under the skin. Migrates from skin to lungs and GI tract | Transmission – barefoot walking in contaminated soil.Treat with ivermectin |
| Trichinella spiralis | Trichinosis | Abdominal pain, nausea, diarrhea for stage 1.Then myositis, fever, eosinophilia and periorbital edema in stage two | From eating undercooked pork. Encysted larvae in muscle. In Central and South America. |
| Toxocara canis | Visceral/ocular larva migrans | Migrating larvae cause inflammation | Acquired from dog feces. Child with vision loss in an endemic country (tropical regions) |
Cestodes (Tapeworms)
| Organism | Disease | Key Features | Clinical Pearls |
| Taenia solium | Neurocysticercosis, seizures | Headaches, vomiting, altered mental status. MRI with cysts | From undercooked pork or ingesting eggs. Brain calcifications on imaging. Treat with albendazole + steroids |
| Diphyllobothrium | Fish tapeworm | B12 deficiency → megaloblastic anemia | Usually asymptomatic. |
| Echinococcus granulosus | Hydatid cyst disease | Liver cysts on US or CT scan, “eggshell” calcifications | Risk of anaphylaxis if cysts rupture. From dog feces. Treat with albendazole, may require surgery |
Trematodes (Flukes)
| Organism | Disease | Key Features | Clinical Pearls |
| Schistosoma spp. | Schistosomiasis | Hematuria (S. haematobium), hepatosplenomegaly, portal HTN (S. mansoni) | 3 types – urinary, intestinal and hepatic Schistosomiasis |
| Clonorchis sinensis | Biliary tract disease, cholangiocarcinoma | Causes pigmented gallstones. | Mostly in Asian countries |
Fungal Infections
Fungal infections can range from superficial skin infections to life-threatening systemic disease, especially in immunocompromised patients.
Common Culprits
Common culprits include Candida, Aspergillus, Cryptococcus, and Histoplasma. These organisms can be yeasts, molds or dimorphic fungi (yeast in the body, mold in the environment).
Clinical Presentation
While some fungal infections may present with similar symptoms in patients, a key differentiating factor will be exposure risk factors such as location (Southwest Desert Fever), activity (spelunking) or immune status of the host.
Superficial infections often cause skin, hair, or nail issues, while invasive fungi may affect the lungs, brain, or bloodstream.
Diagnosis & Treatment
Diagnosis relies on clinical suspicion, history and recent travel, fungal cultures, and histopathology.
Antifungals like fluconazole and harsher treatments like amphotericin B, are the mainstays of treatment. Always consider fungal infections in patients with persistent fevers, neutropenia, or unusual imaging findings as early recognition can be life-saving.
1. Systemic Mycoses
| Fungus | Disease | Key Features | Clinical Pearls |
| Histoplasma capsulatum | Histoplasmosis – intracellular yeast. From bird/bat droppings (caves, spelunking) | Cough, fever, symptoms resemble TB. | MS and OH river valley |
| Blastomyces dermatitidis | Blastomycosis – lung, skin, bone and GU involvement | Broad-based budding yeast | Central and Southern states, MS/OH River Valley. Great Lakes region. |
| Coccidioides immitis | Coccidioidomycosis (Valley Fever) – pulmonary symptoms that can progress to meningitis | Spherules filled with endospores | Southwestern US. “Desert fever” |
2. Opportunistic Fungi
| Fungus | Disease | Key Features | Clinical Pearls |
| Candida albicans | Oral thrush, vaginitis with thick cottage cheese discharge, endocarditis in IVDU | Germ tubes at 37°C, pseudohyphae | Common in diabetics, immunocompromised, and after antibiotic usage. |
| Aspergillus fumigatus | Allergic bronchopulmonary aspergillosis, aspergilloma | Septate hyphae, acute angle branching, eosinophilia | + fungus ball. Treat with voriconazoleneutropenia-associated invasive disease. |
| Cryptococcus neoformans | Meningitis in HIV | Thick polysaccharide capsule | India ink +. From pigeon droppings. |
| Mucor & Rhizopus spp. | Mucormycosis | Irregular, broad, non-septate hyphae at 90° | Diabetics with DKA. Invades nasal → brain. Surgical emergency! |
| Pneumocystis jirovecii | PCP pneumonia in HIV+ patients | Ground-glass opacities; silver stain | CD4 < 200. TMP-SMX prophylaxis in HIV patients. |
3. Cutaneous & Superficial Fungi
| Fungus | Disease | Key Features | Clinical Pearls |
| Dermatophytes (Trichophyton, Microsporum, Epidermophyton) | Tinea infections (corporis, pedis, capitis) | Branching septate hyphae on KOH | Named by body region. “Ringworm” is misnomer—it’s a fungus! |
| Malassezia furfur | Tinea versicolor | Spaghetti-and-meatballs appearance | Lipophilic yeast. Causes hypopigmented patches. |
Tips for Med Students Studying Parasites & Fungal Infections
When studying parasites and fungi, focus on transmission routes, geography, host immune status, especially for opportunistic infections. Matching region to fungal infection (e.g., Coccidioides in the desert Southwest) or symptom to parasite (e.g., anal itching in kids = Enterobius) will help you breeze through vignettes.
Pay attention to lab diagnostics like silver stains, KOH prep, and serology. Memorize which infections require prophylaxis in immunocompromised patients—these frequently show up on exams.
For parasites, differentiate protozoa (GI, CNS) vs. helminths (GI + eosinophilia). Know life cycles and vectors for protozoa and helminths as this can be test fodder for Step 1. Lastly, be confident reading KOH preps, silver stains, and acid-fast tests.
Further Reading
We know—we barely scratched the surface. But that’s the challenge when aiming for the highest-yield coverage in limited space!
For even more microbiology topics, check out these other posts on the blog:
