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The full episode, in writing.
Harrison’s PodClass began in 2019, offering a distinct approach to internal medicine education through audio, led by Dr. Cathy Handy Marshall and Dr. Charlie Wiener. Both hosts are affiliated with The Johns Hopkins School of Medicine, bringing years of clinical and teaching experience to each episode. The podcast is produced by McGraw Hill and is accessible on platforms including Apple Podcasts and iHeartRadio.
Each episode of Harrison’s PodClass centers around case-based discussions drawn from Harrison's Review Questions and chapters from Harrison's Principles of Internal Medicine. This structure allows listeners to hear real-world scenarios unfold, with the hosts dissecting the clinical reasoning process in detail. The format makes the material applicable for medical professionals preparing for board exams, as well as for those seeking to deepen their grasp of complex internal medicine topics.
One episode explores the challenges of infectious diseases in travelers, drawing from chapters in Harrison's Principles of Internal Medicine. Infectious diseases encountered during travel differ significantly depending on destination, activities, and the traveler’s underlying health. For example, malaria risk varies by geography, with sub-Saharan Africa, parts of South America, and South Asia representing high-risk regions. This is due to the prevalence of Anopheles mosquitoes and local transmission of Plasmodium species.
The mechanism behind malaria transmission involves a mosquito bite that injects Plasmodium sporozoites into the bloodstream. These parasites invade liver cells, replicate, and then infect red blood cells, causing cycles of fever, chills, and anemia. In many case vignettes, the hosts highlight the importance of thorough travel history — such as recent exposure in endemic regions or lapses in prophylactic medication — as missed details can delay diagnosis.
Travel medicine also covers diseases like typhoid fever, which occurs most frequently in South Asia. This disease is caused by Salmonella enterica serotype Typhi, which is transmitted through contaminated food or water. The bacteria invade the intestinal mucosa, enter the bloodstream, and can lead to high fever, abdominal pain, and complications such as intestinal perforation if not treated promptly. Harrison’s PodClass episodes often prompt listeners to consider preventive strategies, including vaccination and safe eating practices abroad.
An episode focusing on mosquito-borne illnesses discusses dengue fever, which is prevalent in over 100 countries, especially in urban and semi-urban areas of Southeast Asia and Latin America. Dengue virus, transmitted by Aedes mosquitoes, causes symptoms ranging from mild fever to severe hemorrhagic manifestations. The hosts review the importance of patient education about using insect repellent with at least 20% DEET and wearing long-sleeved clothing. The chemical DEET disrupts the mosquito’s ability to detect human scent by blocking specific olfactory receptors, reducing the likelihood of bites.
Episodes also address diseases like schistosomiasis, common in travelers who swim in freshwater in endemic areas in Africa, the Middle East, and parts of South America. This parasitic infection results from skin exposure to cercariae, the larval form of Schistosoma released by infected snails. After penetrating the skin, the parasites migrate to blood vessels, where they mature and lay eggs, causing symptoms such as rash, fever, and later, organ damage due to the immune response to these eggs.
The podcast delves into practical preventive advice for travelers, emphasizing the need for up-to-date vaccinations. For example, yellow fever vaccine is mandatory for entry into several countries in Africa and South America. The yellow fever virus is transmitted by Aedes and Haemagogus mosquitoes, and the vaccine uses a live-attenuated virus that stimulates an immune response, creating long-term immunity in most recipients after a single dose.
A memorable case vignette may involve a traveler returning from Southeast Asia with a fever and no specific localizing symptoms. The hosts guide listeners through a differential diagnosis that includes diseases such as leptospirosis, scrub typhus, and Japanese encephalitis. Leptospirosis results from contact with water contaminated by urine of infected animals, often rodents. The bacteria invade through cuts or mucous membranes, causing a biphasic illness with fever, muscle pain, and, in severe cases, liver and kidney failure.
Episodes tackling travel-acquired diarrheal illnesses highlight that up to 60% of travelers to low- and middle-income countries develop travelers’ diarrhea. The most common cause is enterotoxigenic Escherichia coli, which produces toxins that increase secretion of electrolytes and water into the intestines. The result is acute onset of watery diarrhea, sometimes accompanied by cramps and vomiting. Preventive advice includes drinking only boiled or bottled water and avoiding raw fruits and vegetables unless peeled.
The podcast frequently emphasizes patient education as a core component of travel medicine. The hosts discuss the value of pre-travel counseling, which includes tailoring advice to the traveler’s itinerary, planned activities, and personal risk factors. For instance, immunocompromised individuals may need additional precautions, such as avoiding live vaccines and taking extra care to avoid contaminated water or soil. Immunocompromised travelers face higher risks because their weakened immune systems are less able to mount effective defenses against pathogens commonly encountered abroad.
In an episode covering post-travel evaluation, listeners are introduced to the approach for undifferentiated fever. The hosts guide the audience through key questions about onset, exposures, and specific risks, pointing out that diseases like malaria can present weeks after return, while others, like dengue, usually have a shorter incubation period. The ability to recognize subtle differences in timing and presentation can be life-saving, as delayed diagnosis of conditions such as malaria can rapidly lead to severe complications, including cerebral malaria and multi-organ failure.
The series extends into rare but critical topics, such as rabies post-exposure prophylaxis. Rabies is nearly always fatal once symptoms develop, but timely post-exposure vaccination, along with administration of rabies immune globulin, can prevent illness. The hosts stress that animal bites abroad, especially in regions where rabies is still endemic, require immediate medical evaluation. Rabies virus travels along peripheral nerves to the central nervous system, causing encephalitis, agitation, and eventually respiratory failure.
Episodes also explore environmental hazards that intersect with infectious diseases. For example, altitude sickness combined with respiratory infections can complicate the clinical picture for travelers visiting high-altitude destinations in South America or Asia. The reduction in oxygen at high altitude impairs the immune response and can exacerbate underlying lung conditions, leading to more severe or prolonged illness when respiratory infections are contracted.
The podcast highlights the importance of understanding local patterns of antimicrobial resistance. In regions where antibiotic-resistant bacteria are common, empirical treatment for infections such as urinary tract infections or pneumonia may need to differ from standard guidelines. This adaptation requires clinicians to remain informed about geographic trends and to educate patients about the need to complete prescribed medication courses, even if symptoms resolve early.
Harrison’s PodClass episodes often encourage listeners to consult the corresponding chapters in Harrison's Principles of Internal Medicine for further reading. This approach reinforces the importance of integrating up-to-date textbook knowledge with clinical reasoning and patient communication skills.
Recent episodes in 2026 have focused on complications from renal dysfunction and hypercalcemia, reflecting the podcast’s commitment to covering both classic and emerging topics within internal medicine. The series remains a valued resource for clinicians worldwide, offering insights grounded in real cases and the enduring legacy of Harrison's Principles of Internal Medicine.