There's been a lot of talk lately about the man who recently came down with ebola in Dallas, Texas. While I live elsewhere, I keep hearing quotes from CDC and state officials that ebola is not contagious until a person shows symptoms. This is generally used in the context of "Don't worry too much; we have the situation under control." Is this true in all cases? Why?


2 Answers 2


First, confirmation from the World Health Organisation:

The incubation period, that is, the time interval from infection with the virus to onset of symptoms is 2 to 21 days. Humans are not infectious until they develop symptoms.


People remain infectious as long as their blood and body fluids, including semen and breast milk, contain the virus. Men who have recovered from the disease can still transmit the virus through their semen for up to 7 weeks after recovery from illness.

They also state that deceased people may still transmit the virus via body fluids.

As to why the patients become infectious only when symptoms show, there are several related ideas. For example, from the CDC:

Ebola virus enters the patient through mucous membranes, breaks in the skin, or parenterally and infects many cell types, including monocytes, macrophages, dendritic cells, endothelial cells, fibroblasts, hepatocytes, adrenal cortical cells and epithelial cells. The incubation period may be related to the infection route (i.e., 6 days for injection versus 10 days for contact). Ebola virus migrates from the initial infection site to regional lymph nodes and subsequently to the liver, spleen and adrenal gland.

According to Dr. Ray Schilling:

Ebola virus disease has an incubation time of 8 to 10 days where the person may complain of some tiredness, but is not sick yet. This is the time when the Ebola virus multiplies. It paralyzes the immune system and distributes itself through the organ system.

From there, the symptoms and effects of Ebola are fast, according to the article "Ebola Virus Pathogenesis: Implications for Vaccines and Therapies" (Sullivan et al. 2003):

Ebola virus replicates at an unusually high rate that overwhelms the protein synthesis apparatus of infected cells and host immune defenses


Every infectous disease works this way: if you had contact with x particles (virus, bacteria, etc...), then you have y% chance to get infected (depends on your immune system, luck, etc...), so it has a distribution. This is called virulence, and it can be measured with ID50 (infectious dose by 50% of subjects) and LD50 (lethal dose by 50% of subjects) values. According to animal studies the LD50 is very low by ebola, so it is highly virulent, but it highly depends on the mode of transmission. So this virulency is compensated by low transmissibility: people infected with ebola, but not showing symptoms release too small amount (or nothing) of the virus to infect others. By hemorrhagic fever you bleed out of every hole of your body, and that blood contains a lot of viruses, that's when it can infect others. So this is not a "Don't worry too much; we have the situation under control." case, it works this way currently. This can probably change due to mutations.

Symptoms usually begin with a sudden influenza-like stage characterized by feeling tired, fever, pain in the muscles and joints, headache, and sore throat.1[12][13] The fever is usually greater than 38.3 °C (100.9 °F).[14]

This is often followed by: vomiting, diarrhea and abdominal pain.[13] Shortness of breath and chest pain may occur next along with swelling, headaches and confusion.[13] In about half of cases the skin may develop a maculopapular rash (a flat red area covered with small bumps).[14]

In some cases, internal and external bleeding may occur.1 This typically begins five to seven days after first symptoms.[15] All people show some decreased blood clotting.[14] Bleeding from mucous membranes or from sites of needle punctures is reported in 40–50% of cases.[16] This may result in the vomiting of blood, coughing up of blood, or blood in stool.[17] Bleeding into the skin may create petechiae, purpura, ecchymoses, or hematomas (especially around needle injection sites).[18] There may also be bleeding into the whites of the eyes. Heavy bleeding is uncommon and if it occurs is usually within the gastrointestinal tract.[14][19]

Recovery may begin between 7 and 14 days after the start of symptoms.[13] Death, if it occurs, is typically 6 to 16 days from the start of symptoms and is often due to low blood pressure from fluid loss.2 In general, the development of bleeding often indicates a worse outcome and this blood loss can result in death.[12] People are often in a coma near the end of life.[13] Those who survive often have ongoing muscle and joint pain, liver inflammation, and decreased hearing among other difficulties.[13]

The time between exposure to the virus and the development of symptoms of the disease is usually 2 to 21 days.1[10] Estimates based on mathematical models predict that around 5% of cases may take greater than 21 days to develop.[11]

The LD50 of mouse-adapted EBO-Z virus inoculated into the peritoneal cavity was ~1 virion. Mice were resistant to large doses of the same virus inoculated subcutaneously, intradermally, or intramuscularly. Mice injected peripherally with mouse-adapted or intraperitoneally with non-adapted EBO-Z virus resisted subsequent challenge with mouse-adapted virus.

Although outbreaks of Ebola virus have largely been confined to endemic regions, their high fatality rate, ability to transmit person-to-person, and low lethal infectious dose make Ebola virus a dangerous threat to public health and pose a great risk for researchers working with these viruses as well as health care personnel treating patients during outbreaks. Furthermore, their potential to be developed into aerosolized biological weapons also causes grave concern for their use as a bioterrorism agent ( Bray, 2003).

Some interesting story about the topic (others were not so lucky):

In 2004, a virologist at USAMRIID was working in a BSL-4 laboratory with mice that had been infected 2 days before with a mouse-adapted variant of the Zaire species of Ebola virus (ZEBOV) (2). The virulence and infectious dose of this variant of ZEBOV are unknown in humans; wild-type virus has a case-fatality rate of up to 90% (3).

The person had been following standard procedure, holding the mice while injecting them intraperitoneally with an immune globulin preparation. While the person was injecting the fifth mouse with a hypodermic syringe that had been used on previous mice, the animal kicked the syringe, causing the needle to pierce the person’s left-hand gloves, resulting in a small laceration. The virologist immediately squeezed the site to force the extravasation of blood. After decontamination of the blue suit in the chemical shower, the injured site was irrigated with 1 liter of sterile water and then scrubbed with povidone-iodine for 10 minutes.

In terms of exposure risk, the needle was presumed to be contaminated with virus-laden blood, although it was suspected that low levels of virus were present on the needle. The animals had not yet manifested signs of infection, and much contamination may have been removed mechanically when the needle pierced the gloves. The local decontamination of the site also reduced potential for infection.

USAMRIID medical, scientific, and executive staff concluded that the person with potential exposure warranted quarantine in the MCS. Contact plus airborne precautions (gown, gloves, N95 mask, eye protection) were used, with a plan to upgrade to BSL-4 precautions for signs or symptoms of illness. These extra precautions were instituted while the patient was asymptomatic for several reasons: 1) the timing of initial clinical manifestations with regard to potential for shedding virus were not known for this specific isolate in human infection; 2) there was interest in ensuring all infection control procedures were being followed appropriately in advance of clinical illness; and 3) there was interest in reducing any potential confounders, such as a caregiver transmitting a febrile respiratory infection to the patient, which might lead to unnecessary procedures or additional isolation. The person was monitored for routine vital signs; daily laboratory studies (coagulation studies, blood counts, chemistries, viral isolation, D-dimer) and regular physician assessments were performed.

Over the next several days, discussions were held with several internationally recognized filovirus experts regarding potential treatments or postexposure prophylaxis options. Local and state public health officials were also notified. The consensus opinion was that there was no safe, readily available source of immune plasma and little evidence existed to support its use. Emergency investigational new drug (IND) protocols were established for treatment with recombinant nematode protein (rNAPc2) and antisense oligomers, with the intention to consider implementation only if the patient demonstrated evidence of infection.

Ultimately, none of the 5 mice had confirmed viremia at the time of the incident. The patient did not become ill or seroconvert and was discharged after 21 days.


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