Did you hear about the folks in New York who got quarantined isolated on the Emirates plane from Dubai?

News like that and folks getting exposed to other infectious diseases, probably has them wondering just how contagious these diseases are. Do you have to be sitting next to someone to get them? In the same row? On the same floor?

Understanding Your Risk of Catching a Disease

Fortunately, most diseases are not terribly contagious.

We worry about some things, like SARS and Ebola, because they are so deadly, not because they are so contagious or infectious.

Wait, contagious or infectious? Aren’t they the same thing?

To confuse matters, some infectious diseases aren’t contagious, like Lyme disease. And some vaccine-preventable diseases are neither infectious nor communicable. Think tetanus. You may have never thought of it that way, but you aren’t going to catch tetanus from another person. Of course, that’s not a good reason to skip getting a tetanus shot!

To understand your risk of getting sick, you want to understand a few terms, including:

• infectious disease – a disease that can be transferred to a new host
• communicable – an infectious disease that can be transferred from one host to another
• non-communicable – a non-infectious disease which can not be transferred from one host to another
• contagiousness – an infectious disease that is easily transferred from one person to another
• infectivity – the ability of an infectious agent to cause an infection, measured as the proportion of persons exposed to an infectious agent who become infected. Although this doesn’t sound much different from contagiousness, it is. The Francisella tularensis bacteria is highly infectious, for example, to the point that folks exposed to a culture plate are given antibiotics or put on a fever watch. Few of us get tularemia though, because transmission is through tick bites, hunting or skinning infected rabbits, muskrats, prairie dogs and other rodents, or inhaling dust or aerosols contaminated with F. tularensis bacteria. So if you get exposed, you will probably get sick, but there is a low probability for getting exposed.
• incubation period – the time it takes to start having symptoms after you are exposed to an infectious disease. A longer incubation period increases the chances that someone will get exposed to a disease and travel home before getting sick. A shorter incubation period, like for influenza, means that a lot of people can get sick in a short amount of time.
• contagious period- the time during which you can spread the illness to other people and may start before you have any symptoms
• quarantine – used to separate people who have been exposed to a contagious disease and may become sick, but aren’t sick yet
• isolation – used to separate people who are already sick with a contagious disease
• transmission – how the disease spreads, including direct (direct contact or droplet spread) vs indirect transmission (airborne, vehicleborne, or vectorborne)
• R₀ (r nought) – the basic reproductive number or the number of new infections originating from a single infectious person among a total susceptible population
• R_n – the net reproductive number, which takes into account the number of susceptibles in a community
• infectious period – how long you are contagious

Got all that?

How Contagious Is Measles?

If not, understanding how easily you can get measles should help you understand all of these terms.

Measles is highly contagious, with a very high R₀ number of 12 to 18.

That’s because:

• the measles virus can live for up to two hours on surfaces and in the airspace where an infected person coughed or sneezed
• infected people are contagious for up to four days before they have a rash and even know that they have measles, so expose lots of people even if they get put in isolation once they get diagnosed
• infected people continue to be contagious for up to four days after the rash appears, so can continue to expose people if they aren’t put in isolation

So you don’t need to have someone with measles coughing in your face to get sick. If they coughed or sneezed at the grocery store, on the bus, or at your doctor’s office and then you entered the same area within two hours, then you could be exposed to the measles virus and could get sick.

Why don’t we see at least 12 to 18 people in each measles outbreak anymore?

That’s easy. The definition for R₀ is for a total susceptible population. Most folks are vaccinated and protected, so even if they are around someone with measles, they typically won’t get sick.

Still, up to 90% of folks who aren’t immune and are exposed to measles will catch it. That includes infants too young to be vaccinated, kids too young to be fully vaccinated, and anyone who has a true medical exemption to getting vaccinated.

The measles has a very high R₀ is easier to see when you compare it to those of some other diseases

 

Infection	R0
Diphtheria	6-7
Ebola	1.5-2.5
Flu	1.4-4
MERS	2-8
Mumps	4.7
Pertussis	5-17
Polio	2-20
RSV	3
SARS	2-5
Smallpox	5-7
Varicella	8-10

Why such a big range for some diseases?

These are estimates and you are more or less contagious at different stages of each illness.

Fortunately, in most cases you can just get vaccinated and protected and don’t have to worry too much about them.

More on the Contagious Periods of Diseases

• The incubation period of a viral infection
• Contagious Period Chart
• CDC – Understand How Infectious Diseases Spread
• WHO – Transmission of communicable diseases on aircraft
• Choose words carefully when writing about Ebola
• What is the difference between infectivity, pathogenicity, and virulence?
• Understand the Measles Outbreak with this One Weird Number
• Why quarantine for measles is critical…and quarantine for Ebola was not
• CDC – Measles (Pinkbook)
• Notes On R0
• CDC – Manual for the Surveillance of Vaccine-Preventable Diseases
• WHO – Infections and infectious diseases
• CDC – Natural History and Spectrum of Disease
• CDC – Have You “Herd”? Modeling Influenza’s Spread
• CDC – Spreading Sickness in Middle School
• Study – Frequent Travelers and Rate of Spread of Epidemics
• Study – Comparison of the Reproductive Numbers of Middle East Respiratory Syndrome Coronavirus Nosocomial Outbreaks in Saudi Arabia and Korea
• Study – Retrospective Parameter Estimation and Forecast of Respiratory Syncytial Virus in the United States