The 10 Most Contagious Diseases that Exist

The world's most contagious diseases are rarely fatal. The most successful viruses find an excellent balance, as they fail to kill their host and are capable of spreading at a dizzying rate.
The 10 Most Contagious Diseases that Exist
Samuel Antonio Sánchez Amador

Written and verified by el biólogo Samuel Antonio Sánchez Amador in 10 September, 2021.

Last update: 10 September, 2021

As indicated by information sources, 95% of the world’s population has some type of health condition. There are more than 7800 million human beings on Earth, and pathogens find in our species an easy reservoir to colonize: resistant, abundant and concentrated in specific points. Find out with us the 10 most contagious diseases that exist.

When it comes to talking about infections and epidemiological figures, we cannot be guided by informational biases or by effectism, since most of the most communicable conditions are the least deadly. Therefore, we’ll bring you the diseases that cause the most infections based on the basic reproductive rhythm (R0), an objective parameter of great importance in the world of virology.

The basic reproductive rhythm (R0) and the most contagious diseases

Before entering fully into the 10 most contagious diseases in the world, it is necessary to provide some background at an epidemiological level. We’re going to focus on the basic reproductive number (R0), as it is an excellent measure of the expansion capacity of a viral, bacterial, protozoal agent, or any type of pathogen.

The basic reproductive rate is defined as the number of secondary cases of a disease caused by a sick patient in a fully susceptible population. In other words, it reflects the number of direct infections caused by a patient from the time they contract the disease until they heal, counting the incubation time and asymptomatic phases.

The formula with which the parameter is constructed is the following:

R0 = τ · c · d

In this operation, τ is the transmissibility of the pathogen, or probability of infection when coming into contact with a patient; c is the contact ratio between sick and susceptible individuals, and d is the duration of infection in the original patient. As you can see, to calculate the R0, both environmental factors and factors specific to the harmful microorganism are taken into account.

Thus, if a specific disease has an R0 of 4, we can infer that on average an infected patient infects 4 people from the time they’re infected until they’re cured. With this idea in mind, we’re ready to learn about the 10 most contagious diseases in the world. Do not miss it!

The R0 takes into account the context of the disease, since the spread of a virus in a city and in a rural area are very different.

1. Cold (R0 from 2 to 3)

The most contagious diseases include the cold
The common cold is quite contagious, and fortunately there are usually no complications. Some age groups are more prone to the disease.

The cold has an R0 of 2-3, which means that an infected person can infect an average of up to 3 people before healing. This figure is quite high, so it isn’t surprising that this group of viral infections is so common in the general population. Still, its spreading capabilities pale in comparison to the bottom of this list.

Although more than 200 types of viruses cause colds, rhinoviruses, coronaviruses, adenoviruses, and enteroviruses are the most common suspects. The average adult has 2 to 3 medical conditions of this type per year, while a young child can suffer up to 12 in the same interval. Due to its rapid expansion and transmissibility, it is considered the most common disease on Earth.

The symptoms are well known: nasal congestion, runny nose (rhinorrhea), throat clearing and sneezing. Transmission occurs primarily when a person releases infected aerosols into the environment with a cough or sneeze, but it can also occur by touching contaminated objects. For example, rhinoviruses last up to 18 hours in the environment in their infective form.

It is estimated that in the United States alone there are more than 1 billion colds each year.

2. Diphtheria (R0 of 2.6)

The basic reproductive rate of diphtheria is 2.6 infections for each patient. This disease is caused by the bacterial agent Corynebacterium diphtheriae, whose toxin is synthesized only when the microorganism is infected by a specific bacteriophage (virus) that integrates its genetic material within it.

This disease is transmitted after direct contact with infected lesions or by inhaling particles emitted by a sick person when coughing and sneezing. Most of the conditions are asymptomatic or mild, but as studies indicate, some outbreaks have presented a mortality rate of up to 10% of diagnosed patients.

Symptoms begin 2 to 7 days after infection, but some people are contagious and asymptomatic at the same time. When clinical signs are present, the patient manifests high fever, tremors, fatigue, bluish skin (cyanosis), cough, sore throat, lymphadenopathy, and difficulty swallowing. Within a few days, the bacteria are capable of destroying healthy tissues in the airways.

Thanks to diphtheria vaccines, the number of annual deaths has dropped from 8,000 in 1990 to 3,300 in 2013.

3. HIV (R0 of 4-5)

The basic reproductive rate of HIV is 4-5, so an infected and untreated person who does not take precautions will infect up to 5 people on average. It is very important to emphasize the untreated, since HIV-positive patients who are up-to-date on antiretroviral therapy (ART) present viral loads in the blood so low that they aren’t considered infectious.

HIV, or human immunodeficiency virus, stands out for its ability to weaken the immune system. This pathogen “hijacks” CD4 lymphocytes to replicate and destroys them in the process, gradually decreasing the patient’s response to infections. As HIVinfo indicates, a person is diagnosed with AIDS when their CD4 lymphocyte count is less than 200 / mm³ of blood.

The transmission of this virus occurs in almost all cases when unprotected sexual intercourse occurs, although there are also cases due to contaminated blood transfusions and from mother to child via the transplacental route. This disease has claimed 33 million lives so far and, at this time, an estimated 38 million HIV-positive patients are estimated.

With antiretroviral therapy, a patient with HIV may never reach the stage of AIDS and remain in a chronic but mild state.

4. Smallpox (R0 from 3.5 to 6)

In this section we can cite one of the most contagious diseases of the past, but which no longer affects the population. Thanks to the global vaccination campaign, the last case of natural smallpox infection was diagnosed in 1977 and in 1980 the World Health Organization certified the eradication of the disease from all over the planet.

The R0 of the Variola virus is 3.5 to 6, so that an infected person could infect up to 6 healthy before being cured or dying. There were 2 variants of the disease:

  1. Smallpox major: This was the most serious and common form. It caused a widespread body rash with purulent blisters, very high fever, vomiting, severe back pain, malaise, severe fatigue, and other generalized clinical signs. The overall mortality rate was 30%, although some variants in this group reached a fatality of almost 100%.
  2. Minor smallpox: The symptoms were much milder, but it was a much less common form of the disease. The mortality rate for patients with minor smallpox was estimated at 1%.

Professional sources estimate that, in its last 100 years of existence, this viral disease killed 500 million people. The eradication of smallpox has been a historic milestone for humanity, as it has shown that vaccination is capable of saving millions of lives.

As in most cases, transmission was through inhalation of infected saliva particles.

5. Whooping cough (R0 of 5.5)

We are entering areas of high epidemiological burden, since whooping cough has an average basic reproductive rate of 5.5 new patients for each infected person. It is one of the most contagious diseases of the upper respiratory tract and is caused by the bacterium Bordetella pertussis. Its condition is serious and it can cause permanent disability in babies.

Initial symptoms are similar to the common cold, but very severe coughing spells begin 10-12 days later. The cough is sometimes accompanied by rales and often causes vomiting, momentary loss of consciousness, and even suffocation and death. The clinical picture can last up to 10 weeks, with an average of 6 weeks.

Fortunately, there is also a vaccine against this condition and its incidence has been reduced by 95% in high-income countries. However, as indicated by the Association of Foreign Health Physicians, in 2008 there were still some 16 million cases and 195,000 deaths annually. The vast majority of patients are located in low-income countries.

6. Poliomyelitis (R0 from 5 to 7)

The R0 for polio stands at a figure of 5 to 7, higher than that of all the other contagious diseases mentioned. It is caused by a virus of the enterovirus genus known as Poliovirus (PV), which has specialized in colonizing the gastrointestinal tract. Its incubation time is very high, as it’s estimated at about 20 days with a maximum of 35.

The National Library of Medicine of the United States shows us the route of transmission of this pathogen. It’s worth highlighting, as it’s different from many other viral agents:

  1. Viruses lodge in the intestine and are excreted in the patient’s stools for several weeks after infection.
  2. Transmission is fecal-oral, that is, people become infected by consuming water or food previously contaminated with fecal matter from a patient.
  3. Transmission can also be oral-oral, something that remains in force in infections in high-income countries with adequate sanitation.
  4. The virus is excreted in its peak concentrations 10 days before and after the onset of symptoms.

In most cases, the infection is asymptomatic, but gastrointestinal clinical signs (abdominal pain and vomiting), flu-like symptoms, and certain breathing difficulties may occur in rare cases. 1% of the cases occur with an infiltration of the bacteria in the patient’s nervous system, which greatly aggravates the condition.

There’s a vaccine against polio. In 2019 there were only 175 natural cases, compared to 350,000 in 1988.

7. Rubella (R0 from 6 to 7)

Rubella is an infectious disease caused by the rubella virus, an RNA agent of the Matonaviridae family. It is another of the most contagious diseases that usually occur asymptomatically, since more or less half of the patients don’t even realize they’re infected. When there are clinical signs, a faint pink rash appears on the body.

Complications from this condition are rare, although bleeding problems, testicular swelling, encephalitis, and inflammation of the nerves are possible. The greatest threat occurs in pregnant women, since 20% of those infected end up spontaneously aborting the fetus.

Widespread vaccination has drastically decreased the incidence of rubella, and in America there hasn’t been an endemic case since 2009. However, certain outbreaks have occurred in various regions of the world in recent decades: for example, 15,000 cases were detected in Japan between 2012 and 2014.

Transmission occurs by inhaling droplets emitted by a sick person. A mother can also pass the virus to her unborn child through the bloodstream.

8. Mumps (R0 from 10 to 12)

We now take a quantum leap on the list, as this is one of the most contagious diseases in the world by far, since an average patient infects 10 or 12 people before they are completely cured. It is a condition located in the parotid glands that is caused by Mumps ortho rubulavirus, a Paramyxoviridae. It’s much more common in children and adolescents.

As studies indicate, 90% of the world’s adult population had been infected before the development of the mumps vaccine, with epidemiological peaks between 2 and 15 years of age. Up to 30% of patients are asymptomatic, but when clinical signs appear, they usually present with fever, jaw pain, malaise, and characteristic facial edema.

Mumps continue to occur sporadically, especially in young children and daycare centers.

9. Chickenpox (R0 from 10 to 12)

With an R0 the same as mumps, chickenpox is another of the most contagious diseases in the world. This pathology is caused by the varicella-zoster virus and transmission is produced by droplets emitted by a sick person when coughing or sneezing. It’s also believed that contagion can be caused if a person scratches or directly touches the blisters of a patient.

We all know the symptoms of chickenpox: itchy rash, fever, headache, and general tiredness. Although we may believe it isn’t a dangerous condition, it should be noted that in 2015 some 6,400 deaths were detected from this disease. In any case, since 2000, strict vaccination schedules for babies have been established in countries like Spain to combat it.

The first dose of the vaccine is given at 15 months of age and the second at 3-4 years.

10. Measles (R0 from 12 to 18)

The most contagious diseases include measles
In addition to being a highly contagious disease, measles is characterized by the presence of multiple skin lesions that mainly affect the trunk.

We have now reached the final disease, which is undoubtedly considered the most contagious disease in the world. Each person with measles can infect up to 18 inhabitants since they are infected with the causal Morbillivirus, a figure that far exceeds other diseases that are supposedly easier to transmit (such as enteric viruses, with an R0 of 17).

The incubation period for measles is 4 to 12 days and patients are contagious even without clinical signs. This is the key to the virus, because thanks to its ability to “go unnoticed” it can be transmitted to up to 90% of people who have come into contact with the patient without knowing it. Like almost all of the above conditions, it is transmitted through the respiratory tract.

Common symptoms are fever, conjunctivitis, cough, and generalized skin rash. It isn’t a fatal disease, but if the majority of the population weren’t immunized from childhood thanks to vaccines, it would be a very serious public health problem. Since 2016, the American continent has been free of measles. The rest of the countries in the world seek the same objective.

Despite its eradication attempts, certain outbreaks have taken place. One of the most notorious occurred in the United States in 2014, with 644 cases in 27 states of the country.

Vaccination is the solution

The analysis of the world’s most contagious diseases and their dynamics deserves pages and pages of scrutiny. It’s noteworthy that most highly infectious diseases aren’t lethal, as experts postulate that the viruses themselves “model” their pathogenesis and decrease it over time in order to spread more efficiently and inadvertently in the population.

Exploring these topics is fascinating from a biological (and medical) point of view, because with them, we see that human beings are in a constant fight against pathogens: they evolve and we generate vaccines. Thanks to vaccination schedules, we have won the battle against polio and smallpox, but there is still a long way to go.

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