I kind of chuckle at all the Ebola hysteria out there. so let’s put some numbers to this outbreak to show why

- if you live in the USA
- Don’t travel to West Africa
- And don’t work in the health care system…

…you won’t get Ebola (300 million Americans fit this profile). I’ll even calculate worst case scenarios to show that even in the worst case, you have nothing to worry about.

WARNING: This isn’t going to be some fancy journalistic writing…it’s my back of the napkin calculations that I did over two slices of pizza, and one hour my wife was at the grocery store.

WARNING 2: Because these are back of the napkin calculations, I use some unfounded assumptions. But each unfounded assumption is likely worse-than-worst case scenario.

First lets start with something called the basic reproduction number of an infectious disease, which we designate, R_{0}. To keep it simple, this number represents the average number of uninfected people infected by someone else who has a certain disease. In any given population:

- if R
_{0} is greater than 1 then a disease will continue to spread throughout said population
- If R
_{0} is less than 1 then the disease will eventually die out in that population

For Ebola, the value of R_{0} in the West African Ebola outbreak is between about 1.7 and 2. This means, if things don’t improve, the disease will continue to spread within that population.

But…

That’s within the West African population (and even then mainly confined to Guinea, Sierra Leone, and Liberia). Think about that. These people have poor medical care, often Ebola patients still intermingle with other people (usually family members), and often do not safely bury the deceased. Even with all that, one person only infects 1.7-2 people on average!

Since most of you reading this are in developed countries this R_{0} number doesn’t apply to your population.

Let’s think about this. So far we’ve had eight cases of Ebola in the United States. This includes five people who were evacuated to the United States AFTER being diagnosed. So far, four cases have concluded. Three people were discharged after Ebola ran its course, and one person died. That’s a fatality rate of 25%. Four other cases are still being treated.

If we just take the cases that originated in the United States, then we have three cases.

- One person contracted Ebola in Liberia, flew to the United States, then developed symptoms, was diagnosed, treated, and died.
- Two health care workers who attended to the first patient have since contracted Ebola, and both are currently receiving treatment.

A few observations about this:

- Nobody who was on the plane with the original (also called index) case got Ebola
- The only people who contracted Ebola were health care workers who treated the original patient
- So far, we have one completed case of Ebola from these three people, and two others infected from the index case, so R
_{0} is 2
**IF** neither of these other two people infect another person, then R_{0} would drop to 0.67, and the disease will have died out within the United States (for this index case)
**IF** we include the completed cases of those brought to the United States after having contracted Ebola, then the current R_{0} within the United States is 0.5 (two people within the United States have been infected from four completed cases)
**IF** all current cases treated or diagnosed within the United States complete with no transmissions, then R_{0} will be 0.25 (two transmitted cases within the United States out of eight total cases)

I understand those last three bullets are hypothetical, but they do show that with good tracking, good procedures, good sanitation, and good medical care, we can get R_{0} below the critical threshold of 1. I feel pretty confident about that, given that in West Africa there is poor tracking, procedures, sanitation, and medical care, yet R_{0} is only between 1.7 and 2.

Let’s dive even deeper. As we know, one of these two remaining cases did in fact travel by airplane around the time of first symptoms (there are reports it was the day before, and newer reports saying there may have been signs earlier). Let’s assume worst case scenario that this person did fly while contagious. Then at worst, out of an abundance of caution, around 800 people that flew on one of the two planes in the time after the subject flew but before the planes were removed from service are being contacted or tracked.

This means at worst:

- 8 patients
- An unknown amount of health care workers
- And 800 people associated with the flights of the traveling patient…

…are the sum total of possible exposures. If we round this up to 1000 to account for the unknown amount of health care workers, that means about 0.0003% of the total United States population has maybe been exposed to Ebola.

Of those 1000 people, the way for them to contract Ebola would be for them to have come in contact with the bodily fluids of an infected person. I don’t know about you, but I typically don’t come into contact with someone else’s bodily fluids on an airplane. Let’s assume that of these 800 people that flew with or after the third Ebola patient flew, that 25% of them sat in the vicinity of the patient and/or used the bathroom after the patient did. Now we’re talking 200 people, plus the additional 200 health care workers we estimated from rounding. Now we’re talking 0.00125% of the population possibly being exposed.

Even then, we know not all of these people would come into contact with the patient’s bodily fluid and contract Ebola. So far, out of our assumed 200 health care workers, 100 have been from completed cases, and we have 2 transmissions for a rate of 1 in 50 (2%). Let’s assume 2% of our 400 people get Ebola, then we have a total of 8 more cases.

That would bring us to a grand total of 16 cases out of 320,000,000 people, or 0.000005% of the population, or 1 in 20 million.

Of those, let’s take the worst case estimate of the case fatality rate for the current outbreak of 70%. Then out of all those exposed to Ebola so far, approximately 12 people would die from the current Ebola outbreak within the United States, or 1 in 26.7 million.

Given the original 6 cases of Ebola, there would have been 10 more cases for an R_{0} of 1.67. That’s for a worst case scenario! Our worst case scenario is less than the best case scenario for West Africa!!!

But think about this…these eight cases so far have all been from people who were in West Africa, or a health care worker who treated someone who was in West Africa. Of the other assumed cases, we assumed it would be split half and half between health care workers and general population (those on the plane). So only four, yes, FOUR out of 300 million non-health care workers, non-West Africa visitors, would in the worst case contract Ebola in the month since it was first diagnosed within the United States. That’s 1 in 75 million people. And of those, only three would likely die, meaning **worst case 1**** in 100 million Americans in the general population might die as the third generation of transmission of Ebola within the United States.** Any further deaths would either be from a new index patient, or from the third generation of this current case passing it on to a fourth generation.

So, for the three cases of Ebola where the people were out in the general public (the original case diagnosed in the United States, plus the two people he transmitted it to), only FOUR people in the general population as a worst case scenario will contact Ebola. That puts the R_{0} at 1.33. And if we did even 1 person better than that, suddenly the R_{0} is at the magic number of 1 (meaning it wouldn’t die out, but at any given time the average number of people with Ebola would be the same).

And again…I’m almost assuredly severely over-estimating these chances, so that I can present the worst case scenario. So therefore, I’m almost assuredly saying we’ll be at the R_{0} value of 1 or less in the general population, and this current episode will get snuffed out within the general population, until the point we can get it snuffed out from the health care/West African traveling population (which will only happen when the current outbreak in West Africa ends).

So, there may be a few handfuls of times Ebola crops up in the United States until the current West African outbreak is fully eradicated. But in each case, if you’re in the general population, you have a minuscule chance of catching Ebola. Something like 1 in tens of millions. And your chances of dying from it are even less.

So we’ve had 4 completed potential index cases in the United States in a 3 month span, with 2 more index cases receiving treatment. That’s two per month. However, that rate will almost surely pick up simply because the rate is still increasing in West Africa. But even if we assume worst case an average of 10 new index cases per month, that’s 120 per year. So if we’re at the R_{0} value of 1 or less, as we can safely assume, then we may see 360 cases in the next year in the general American population, and 250 deaths. That’s greater than a one in one million chance of you dying from Ebola within the next year. Again, that’s using worst case death rates, a worst-case projection of the number of new cases in the next 12 months, and a near worst case projection of transmission of these cases to the general public.

To put it simpler, even in the worst case scenario, if you’re in the general population then you have about twice the chance of getting struck by lightning in a given year than dying from Ebola…and you don’t worry about getting struck by lightning, do you?

I’m not losing any sleep over it, or giving it a single worry. I suggest you do the same. You’ll enjoy life more by not worrying!

-Nick

PS: This