SARS typically starts looking like a bad flu, with a high fever above 38°C (100.4°F), chills, headache, and muscle pain. Within a few days, it shifts into a respiratory illness with a dry cough and increasing shortness of breath. On chest imaging, SARS produces hazy white patches across the lungs. Under a microscope, the virus itself is a tiny sphere studded with protruding spikes, giving it the crown-like appearance that earned coronaviruses their name.
Early Symptoms: The First Week
After an incubation period of 2 to 7 days (occasionally up to 10 or even 14 days), SARS announces itself with a sudden high fever. The fever is often accompanied by chills, body aches, headache, and a general feeling of being unwell. At this stage, some people have mild respiratory symptoms, but the illness feels more like a severe flu than a lung infection. Rash, neurological symptoms, and gastrointestinal problems are uncommon, though a small number of patients reported diarrhea during this early febrile phase.
This initial period is deceptive. People feel sick, but the hallmark feature of SARS, the lung involvement, hasn’t fully set in yet. That changes within days.
The Respiratory Phase
Roughly 3 to 7 days after the first symptoms appear, SARS transitions into its lower respiratory phase. A dry, nonproductive cough develops, and breathing becomes progressively more difficult. Blood oxygen levels can drop as the lungs become increasingly inflamed and less efficient at gas exchange. For most patients, this is the most alarming part of the illness, as what began feeling like the flu now feels like a serious chest infection.
In 10 to 20% of cases, the respiratory illness becomes severe enough to require intubation and mechanical ventilation. These patients develop a condition similar to acute respiratory distress syndrome, where the lungs fill with fluid and inflammatory debris, making it extremely difficult to breathe without machine support. Older patients, particularly those over 60, tended to deteriorate faster during the 2003 outbreak, with peak death rates occurring around 12 days after hospital admission, compared to roughly 18 days for younger patients.
What SARS Looks Like on a Chest X-Ray
On imaging, SARS creates a distinctive but nonspecific pattern in the lungs. Early chest X-rays show patchy areas of haziness, called ground-glass opacities, that can appear in one or both lungs. These cloudy regions represent areas where the air sacs are filling with fluid and inflammatory material, partially blocking the passage of X-rays. Some patients also show denser white patches, known as consolidation, where sections of lung tissue have become so saturated that they look almost solid.
As the disease progresses, these findings can spread. In severe cases, imaging reveals extensive bilateral ground-glass opacities throughout both lungs, along with consolidation concentrated in the lower lobes. A case documented in the American Journal of Roentgenology showed a previously healthy 55-year-old man whose initial X-ray revealed widespread haziness in the right lung with relative sparing of the left lung apex, which then progressed to diffuse consolidation in both lungs after intubation. CT scans provide more detail, showing patchy areas of affected tissue interspersed with lobular areas of normal, spared lung. These imaging patterns are not unique to SARS. They overlap with other viral pneumonias and respiratory distress syndrome, which is why imaging alone couldn’t confirm a diagnosis.
What the Virus Looks Like Under a Microscope
The SARS coronavirus is far too small to see with a regular microscope. Under electron microscopy, it appears as a roughly spherical particle between 80 and 140 nanometers in diameter, about a thousand times smaller than the width of a human hair. The most recognizable feature is a ring of spike proteins projecting outward from the virus’s outer membrane. Each spike is 20 to 25 nanometers long, and a typical cross-section of a single virus particle shows about 9 to 10 of these protrusions. These spikes give the virus its “crown” appearance, which is where the name coronavirus (from the Latin “corona,” meaning crown) comes from.
Beneath the spiky membrane sits the nucleocapsid, a tightly bundled package of the virus’s RNA genome wrapped in protective protein. The overall structure is simple: a lipid envelope studded with spike proteins on the outside, and genetic material coiled inside. This architecture is shared across the coronavirus family, including SARS-CoV-2, the virus responsible for COVID-19.
What Blood Tests Reveal
SARS also has a recognizable signature in blood work. The most consistent finding is lymphopenia, a drop in lymphocytes, the white blood cells that fight viral infections. In one study of 76 patients, about 65% had low lymphocyte counts at the time of admission, and nearly 95% developed lymphopenia at some point during their illness. The lowest lymphocyte counts typically occurred around day 7 of the disease.
Low platelet counts were also common, affecting about 46% of patients initially and 80% over the full course of illness. Liver enzymes frequently climbed as well, with elevated levels appearing in roughly 87% of patients for one marker and 78% for another, peaking around days 10 to 13. Elevated markers of inflammation and tissue damage rounded out the picture. None of these findings alone confirmed SARS, but together they formed a recognizable laboratory pattern that helped clinicians distinguish it from other respiratory infections.
How SARS Was Officially Identified
During the 2003 outbreak, the WHO established a two-tier system for classifying cases. A suspected case required a high fever above 38°C plus cough or breathing difficulty, combined with a known exposure: close contact with a confirmed or suspected SARS patient, recent travel to an area with active transmission, or residence in such an area. “Close contact” specifically meant caring for, living with, or having direct contact with the respiratory secretions or body fluids of an infected person.
A probable case went a step further. It required either chest X-ray findings consistent with pneumonia or respiratory distress syndrome, or a positive laboratory test for the SARS coronavirus. This layered approach reflected the reality that SARS looked like many other respiratory illnesses on the surface. Confirming it required combining clinical symptoms, exposure history, imaging findings, and lab results into a complete picture.