Three more Boston residents diagnosed with coronavirus; all contracted it at meeting at Marriott Long Wharf

There's a group of high school students back from Italy who only just went into quarantine.

A public health official just said that *nobody* from any of the cape cod hospitals have been tested: https://www.capecodtimes.com/news/20200305/local-coronavirus-tests-need-...

...and only 25 people from the entire state have been tested.

So that means that likely nobody in that group of dozens of students have been tested, despite them milling around in the general population for a while.

Brilliant.

What? Yes, it is. That's the primary method of infection - breathing airborne droplets generated by an infected person coughing or even laughing.

What? Yes, it is. That's the primary method of infection - breathing airborne droplets generated by an infected person coughing or even laughing.

This virus doesn't seem to infect people who just happen to breath it in from the air -- there are some number of viruses which can do this. Note that: to the best evidence the [SARS-CoV-2] virus [the virus associated with COVID-19 disease] is not an "airborne virus" except in the special case of the virus being a "passenger" on or in an airborne droplet ejected by an infected person who sneezes or coughs. So to be able to be infected by the virus -- either the airborne droplet must either hit you in the face [mucus membranes of the nose, eyes or mouth] or fall onto a surface which you later touch and carry to your face for their to be any opportunity for transmission.

The range of such a droplet is just physics [aerodynamics] the same as for a fast ball thrown by say Chris Sale. The major difference being that the fast ball is for the most part on its own in encountering the air between the mound and home plate. It's velocity decays based on the air resistance which in turn depends on the frontal area of the baseball and to a lesser degree on rotation and surface roughness. In most cases the baseball arrives at the catcher's mitt not much slower than it left the pitchers hand.

The droplet leaves the infected person's nose or mouth traveling quite rapidly entrained in a stream of air. The stream of air carrying the doplets and in turn the viruses encounters stationary air and within a short distance the droplets are moving on their own. However, the droplet is quite low in mass and so the velocity of the droplet rapidly decelerates due to air resistance -- depending on droplet size. Sneezes are initially faster moving but smaller droplet sizes than coughs. Sneezes are also typically directed mostly downward due to the orientation of your nostrils. In either case after a relatively small distance [about an arm's length or so] the droplet is essentially just drifting in the air and depending on size mostly falling. The general assumption is that to be in the state of direct risk of being hit by a droplet you need to be within 2m in a direct line to the infected sneezer or cougher.

Much greater opportunity to contact a surface which could have been contaminated anytime in the past interval since the last cleaning with an 100% type of anti-virus cleanser such as alcohol or bleach.

So what is our risk of catching COVID-19 disease on say a travel on the T to attend some event such as the Spring Flower Show next weekend.

There are still only hundreds of known cases in the entire 320 Millions of us in the entire US and a handful more in Canada all of these are in isolation. In addition the disease trackers are having some few thousands of their contacts self-quarantine.

Suppose that there are 10X the number of identified cases who are not showing any symptoms and don't have any clear case connections -- so perhaps there could be 3,000 "Typhoid Mary's" out there.

The probability of you running into someone in the US carrying the virus while you take the T or attend some local event is the same as there being one of the 3,000 in the circle of people with which you would interact. Will get back to that probability in a few.

So how many people are you likely to interact with at contact distance [less than 2m] in say a T ride involving a bus, two subways and 3 stations -- surprisingly few -- ignoring rush hour and ignoring common things that you touch.

Waiting for the bus outside -- 0
Boarding the bus -- 2 or 3
riding on the bus -- 2 or 3
leaving the bus -- 2 or 3
entering the subway station -- 2 or 3
buying some rides on a Charlie Card machine -- 1
standing on the platform -- 1
entering the first train -- 1
riding on the first train -- 2
exiting the first train -- 1 to 5 depending on how crowded
traversing the station to the 2nd train platform -- 2 or 3
waiting on the platform -- 3
entering the 2nd train -- 2
riding on the 2nd train --2
exiting the 2nd train -- 2
leaving the station --2
walking on the street -- 0
total [hypothetical journey]: 27 to 36 depending on details

Let's assume that the number is 100

So what is the probability that in that 100 people you might encounter there is one of the 3,000 people chosen at random from a pool of people in the US of 300 Million to make the math simple

First we need to calculate the probability of picking one of 3,000 in 300 Million
Well the probability to pick one person out of 300 Million is 1/3x10^8. Now to simplify things let's assume that making 3,000 picks doesn't deplete the pool.

so then the Probability of picking once and getting one of the "hot ones" is 3x10^3/3x10^8
== 1/10^5

However, for each of the 100 you encounter the same probability applies so its like taking 100 picks at the lottery -- your overall probability of encountering an infected person during our hypothetical T trip is 100x (1/10^5) === 1/10^3.

Of course that assumes that at each interaction you have an equal chance of picking from a pool in which 1/10^5 are infected. In reality the cases are mostly concentrated on the West Coast with a secondary cluster in NY metro area. Riding the T is not likely to tap either of those pools.

So let's look locally where we have about 10 cases [all of whom are now isolated -- but we'll assume an equal number undetected so far] in the Greater Boston area of about 5 Million = 10/5x10^6 == 1/5 x10^5. With our hundred picks at the pile we come up with another estimate of the chance of encountering an infected person as 1/5 x10^3.

The ultimate key is then don't stay close to someone who is coughing or sneezing, and wash your hands or use a sanitizer after contacting a surface which could be contaminated -- such as something that you held onto in order to stand-up on the subway.