a mom, and her son on the right.

And it turns out that mom has tuberculosis,

let's assume that.

And sometimes when you see "Tuberculosis"

written out the way I'm writing it out,

you'll actually see it shorthanded,

or kind of use the quick way of saying it,

which is two letters: TB.

So let's say mom has TB.

Now, this is actually a diagnosis, right?

This is a description of her illness.

This is telling us what she actually has,

what she is sick with.

But we have to remember that tuberculosis is

actually caused by an organism.

It's actually caused by a bacteria, it turns out.

And this bacteria has the name "Mycobacterium".

Mycobacterium Tuberculosis.

So this is actually a very easy one to remember,

because "tuberculosis" is right here in the name.

Now I should point out Mycobacterium Tuberculosis

is actually not the only cause of tuberculosis.

It turns out there are few other kind of related

mycobacterium, using this word "myco", that also cause TB.

But this one, the one I wrote out for you,

this is definitely the most common around the world,

and that's the one I'm going to focus on.

And in fact this "myco", this is actually Greek

for the term "fungus".

And the reason that this is here actually kind of tells us

a little bit about how this bacteria grows,

because it grows really slowly, like a fungus,

and that's actually the reason that they use the term "myco".

But nevertheless it is a bacteria, and so if we're going to

put a little bracket around the diagnosis,

I also want to put a little bracket around this part,

to kind of distinguish the two.

So now you can see very clearly TB,

the diagnosis is caused by a bacteria.

So now let's talk about how mom, who we said already

is sick with TB... I'm going to actually just

sketch out what her lungs might look like,

assuming that the TB is in her lungs.

This is actually the most common place we think of

with TB, but not the only place.

But let's say that she's got little red, I'm going to draw it

in red, bacteria here in her lungs, causing her to be

very, very sick with tuberculosis.

She could spread it to her son.

But what are the different ways that she might spread it?

What are the most common ways?

Well, let me sketch out a few possibilities,

and we're going to go over whether these possibilities

are very likely or unlikely to be a way for her

to spread disease to her son.

Let' say first they are sharing this delicious pizza

I'm drawing here. Let's say they are very into pizza

and they like to share food, and they both chow down

on this little pizza here, that's one way they might

potentially you might think of as a way to spread it.

Maybe they're even sharing a drink.

Maybe there is a drink here, and they are sharing again.

You might also think about what's going on

in their house: maybe they're opening and closing doors,

and maybe they're touching door knobs,

there is another way, right, maybe they're touching stuff

in common. Maybe she says to him:

"Hey, here, grab these keys" and she's been holding

the keys all day, and then she gives him the keys

and he holds the keys.

There is another way, maybe the TB can touch objects

in the environment, like a door knob or a key.

And then there's the most obvious way

you might be thinking: maybe she's coughing,

maybe she has a loud cough, maybe she's coughing

all day and some of these bacteria get in the air.

That's another way that you might imagine that the

bacteria could spread from her to her son.

So different ways, right. Now, of these ways,

I'm actually going to label this one over here,

let's say this is through the air.

Which are the most common ways to be really concerned

about TB spreading?

I'm actually going to just put it in green,

so it really sticks out.

The most common way is what we call

"Person to person through the air".

So in this case the first person would be mom,

because she is sick, and it's going to go

through the air, down to her son.

And these other ways, for example food and drink,

that's really not so common, that's really really unlikely

to be a way of spreading TB and in fact,

even this down here is really not likely either.

So the idea of getting TB by sharing food and drink

or touching objects in your environment

like the keys or the door knob, or things like that,

that's really not how TB spreads usually.

Usually it spreads through the air.

And one person, the sick person is usually coughing a lot,

and then the other person might breathe it in.

So let me make a little bit of space on this canvas

and let's talk about what happens next.

I'm going to draw one alveolus here,

and I'm going to copy it a few times just so you can see

a few different possibilities in terms of what might happen,

and these represent the son's alveoli.

These are the son's alveoli.

And of course these are the tiny little air sacks

at the very ends of the bronchial tree, right?

So we'll make a few copies of this.

There we go, we have four possibilities:

Possibility 1, 2, 3 and 4.

Basically, we'll go through different scenarios,

different things that might happen when mom coughs.

So maybe she coughs and the first possibility could be

that the bacteria just don't get far enough,

they don't actually make it to the son,

and he never ends up breathing them in.

So if this was the case, there would be no bacteria

in his alveoli, of course his lungs are nice and clean,

let me draw his lungs in, they look nice and clean,

with no bacteria, and he's feeling great.

Right, this is our son over here feeling really good,

and we would say basically in this case,

in scenario one, he's healthy, because the bacteria

never even got to his lungs.

Now, Scenario 2.

Let's say that the cough actually was very strong

and he was close by and he ended up breathing

some of these in through his nose or his mouth

and they went down into his lungs.

That's another possibility.

Once the bacteria get there, let me actually draw them

on this little alveoli, in possibility number 2,

they might actually get picked up by little immune cells.

So he has little cells that are patrolling the lungs,

making sure they are nice and clean and healthy,

and these little immune cells, we'll label them over here,

these are Macrophages, this literally means

"Big eater", because "phage" means "to eat",

so these immune cells, they might come by and gobble up

these bacteria, and take them in, and destroy them.

That's another possibility.

So that would be possibility number two.

So here the bacteria are gone.

Now let's play it out again.

And let's say in Scenario 3 also you have a couple

of bacteria in here, and just as before, you got a couple

of immune cells that come by, and they swallow up these

little bacteria, these are the macrophages I'm drawing,

swallowing up the bacteria, but let's say that

unfortunately in Scenario 3, now, these macrophages,

for whatever reason, cannot destroy the bacteria.

The bacteria is still living, and that's why I draw them here

as little red dots. They are still living, still there.

And now let me draw the fourth scenario,

which is again let's say a couple of bacteria get in,

and the immune cells again get alerted, and they come by,

and pick up one of them, maybe thîs immune cell is

trying to go after this other one, maybe it's really close by,

but here the key difference is that these bacteria

are actually multiplying, so I'm going to draw lots of them.

These bacteria are multiplying and they're filling up

this space. So this space is filling up with

little tiny red bacteria.

So the key difference here is that

these ones are multiplying.

And we didn't really talk about the other scenarios

having bacteria that are multiplying.

But now, that's the key new thing here.

And in this scenario, we'd call it "Active"...

because you're actually seeing the bacteria thriving,

we call this "Active TB infection".

And that goes back to what we would label

the other scenarios, these ones, and these ones together,

we actually call both of them "Latent TB infection".

And the reason I'm putting them together is because

it's very hard clinically to distinguish Scenario 2

from Scenario 3, because in both cases,

the immune system has previous experience with

the TB bacteria, it's seen the TB bacteria,

and in both cases you're not seeing lots and lots

of bacteria dividing or multiplying,

so we lump these together and call them both

"Latent TB infection".

The real key, and this kind of the take home that I want

to point out, is that there is difference then between

"healthy", someone that's really never seen TB

in their life before; "Latent", where you have seen TB

previously, but you don't have any bacteria

that are multiplying; and "Active TB infection",

where you have lots and lots of

TB bacteria that are multiplying.

Let me make just a little bit more space then.

I'm going to focus now on just this final one,

this multiplying, active TB infection situation.

So if, let's say, our son in this case,

gets tuberculosis from mother, from mom,

and let's say unfortunately he has an

active TB infection, what are some clues

to tell us that he has an active infection?

If I'm trying to figure out if somebody has TB,

I always think about two key things:

What are their symptoms? What are they sick with?

That's the first thing.

And then: How long is it going on for?

I'm going to call that "Duration".

And these two offer really really helpful clues

to figure out if someone has TB.

And with symptoms, I'm going to break it up into

two categories. The first is "Constitutional",

and this is constitutional symptoms, and this is

the things that affect the whole body.

The whole body, so I'm going to put a little bracket

on the entire body to remind us of that.

And this could be things like fevers or chills,

you can't really point to one part of your body

and say "This is the part that's having fevers and chills".

You'd say just generally "I feel awful".

This could be things like night sweats if you wake up

and your t-shirt is all wet, you might say

those are night sweats.

Another example of a constitutional symptom

is weight loss, particularly when you are not trying

to lose weight, especially because you are

maybe not eating as much, or you're vomitting.

Anything like that...

And now the other category is "Lower respiratory tract".

"Respiratory" I'm going to abridge it to "Resp." tract.

And this, if I want to draw it, it would basically be

the part I've drawn in blue here.

So going down from your voice box all the way

to the alveoli. This would be your lower respiratory tract.

And you can think about what sort of symptoms