Step 1 topics → Microbiology → Filoviruses (Ebola, Marburg)

Filoviruses (Ebola, Marburg)

USMLE Step 1 trap: Incorrectly attributes airborne/droplet transmission as the primary route for Ebola virus. Ebola is transmitted through direct contact with blood or body fluids of symptomatic individuals, not via respiratory droplets; airborne precautions are used out of caution but contact/droplet is the primary route.

Filoviruses — Ebola and Marburg — are enveloped, negative-sense single-stranded RNA viruses that cause viral hemorrhagic fever, and USMLE Step 1 tests two common misconceptions about them. First, Ebola does not spread through the air — transmission requires direct contact with blood or body fluids from a symptomatic patient. The aggressive PPE healthcare workers use reflects the severity of the disease and contact/droplet precautions, not airborne transmission. Second, envelope status says nothing about genome polarity — filoviruses are enveloped AND negative-sense, which means they must carry their own RNA-dependent RNA polymerase since their genome cannot be directly translated by host ribosomes. Nail those two points and you have the USMLE Step 1 filovirus content covered.

The exam tests filoviruses from three main angles: structure (what kind of RNA, enveloped vs. not, shape), clinical presentation (hemorrhagic fever pattern, which distinguishes it from other viral syndromes), and management (supportive care, strict isolation, and knowing that an FDA-approved vaccine for Ebola exists). The 'application' angle matters here — they may describe the clinical scenario and ask you to match to the pathogen class, or give you a structural clue and ask you to predict transmission behavior.

Two misconceptions reliably trip students up. First, many students think Ebola spreads like influenza — via respiratory droplets — because of how dangerous it is and how aggressively healthcare workers are isolated. Wrong: transmission is through direct contact with blood or body fluids. Second, students link 'enveloped virus' with 'positive-sense RNA' as a mental shortcut — but envelope status says nothing about genome polarity. Filoviruses are enveloped AND negative-sense, and they must carry their own RNA-dependent RNA polymerase as a result. Nail these two distinctions and you've got the USMLE Step 1 filovirus content locked.

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Common misconceptions

Common mistake
Wrong: Ebola is primarily transmitted via respiratory droplets like influenza.
Right: Ebola is transmitted through direct contact with blood or body fluids of symptomatic individuals, not via respiratory droplets; airborne precautions are used out of caution but contact/droplet is the primary route.
Ebola is NOT airborne — it does not spread through the air the way influenza or measles does. Transmission requires direct contact with blood or body fluids from a symptomatic (not asymptomatic) infected person. Healthcare workers use aggressive PPE including airborne precautions out of an abundance of caution in high-risk settings, but this does not mean airborne transmission is the primary route — contact and droplet exposure are the real risks. Don't let the severity of the disease mislead you into assuming it must spread like flu.
Common mistake
Wrong: Filoviruses have a positive-sense RNA genome because they are enveloped.
Right: Filoviruses (Ebola, Marburg) have a negative-sense single-stranded RNA genome and must carry their own RNA-dependent RNA polymerase.
Being enveloped has nothing to do with genome polarity — these are independent structural features. Filoviruses (Ebola, Marburg) are enveloped AND negative-sense ssRNA viruses. Negative-sense means the genome cannot be directly translated by host ribosomes, so the virus must package its own RNA-dependent RNA polymerase to first convert the genome into a readable positive-sense strand. Think of negative-sense RNA as the 'wrong direction' strand that needs to be flipped before anything can be made from it.
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What the exam tests

  1. Identify the structural characteristics of filoviruses: they are enveloped, filamentous virions with a negative-sense single-stranded RNA genome — and know what that genome polarity implies (must carry their own RNA polymerase).
  2. Recognize the clinical syndrome of hemorrhagic fever — fever, systemic shock, and mucosal bleeding — and associate it with filoviruses (Ebola, Marburg), especially in a traveler returning from sub-Saharan Africa.
  3. Know the management approach: supportive care, strict isolation (contact/droplet precautions primarily), and awareness that an FDA-approved Ebola vaccine (rVSV-ZEBOV) exists.

Can you avoid these mistakes?

A researcher describes a virus as enveloped with a filamentous shape. Does that tell you whether the genome is positive- or negative-sense? What additional fact about filoviruses confirms their genome polarity — and what functional consequence does that polarity have?
A patient returns from the Democratic Republic of Congo with high fever, hypotension, and bleeding from IV sites. You suspect Ebola. What isolation precautions do you implement, and what is the primary route of transmission you are trying to prevent?
A classmate says 'Ebola must be airborne because healthcare workers wear full respirators.' How would you correct this reasoning, and what does the actual transmission data say?
You are given four RNA viruses: influenza, poliovirus, Ebola, and HIV. Which ones must carry their own RNA-dependent RNA polymerase into the host cell, and why? (Hint: think about genome polarity for each.)

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