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In 1796, the scientist Edward Jenner

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injected material from a cowpox virus
into an eight-year-old boy

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with a hunch that this would provide
the protection needed

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to save people from deadly outbreaks
of the related smallpox virus.

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It was a success.

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The eight-year-old was inoculated
against the disease

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and this became the first ever vaccine.

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But why did it work?

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To understand how vaccines function,

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we need to know how the immune system
defends us against contagious diseases

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in the first place.

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When foreign microbes invade us,

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the immune system triggers
a series of responses

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in an attempt to identify
and remove them from our bodies.

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The signs that this immune
response is working

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are the coughing, sneezing,
inflammation and fever we experience,

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which work to trap, deter and rid the body
of threatening things, like bacteria.

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These innate immune responses
also trigger our second line of defense,

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called adaptive immunity.

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Special cells called B cells and T cells
are recruited to fight microbes,

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and also record information about them,

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creating a memory of what
the invaders look like,

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and how best to fight them.

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This know-how becomes handy

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if the same pathogen
invades the body again.

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But despite this smart response,
there's still a risk involved.

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The body takes time to learn
how to respond to pathogens

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and to build up these defenses.

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And even then,

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if a body is too weak or young
to fight back when it's invaded,

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it might face very serious risk
if the pathogen is particularly severe.

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But what if we could prepare
the body's immune response,

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readying it before someone even got ill?

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This is where vaccines come in.

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Using the same principles
that the body uses to defend itself,

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scientists use vaccines to trigger
the body's adaptive immune system,

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without exposing humans
to the full strength disease.

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This has resulted in many vaccines,
which each work uniquely,

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separated into many different types.

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First, we have live attenuated vaccines.

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These are made of the pathogen itself
but a much weaker and tamer version.

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Next, we have inactive vaccines,
in which the pathogens have been killed.

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The weakening and inactivation
in both types of vaccine

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ensures that pathogens don't develop
into the full blown disease.

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But just like a disease,
they trigger an immune response,

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teaching the body to recognize an attack

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by making a profile
of pathogens in preparation.

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The downside is that live attenuated
vaccines can be difficult to make,

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and because they're live
and quite powerful,

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people with weaker immune systems
can't have them,

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while inactive vaccines
don't create long-lasting immunity.

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Another type, the subunit vaccine,

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is only made from one part
of the pathogen, called an antigen,

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the ingredient that actually triggers
the immune response.

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By even further isolating
specific components of antigens,

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like proteins or polysaccharides,

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these vaccines can prompt
specific responses.

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Scientists are now building
a whole new range of vaccines

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called DNA vaccines.

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For this variety, they isolate the very
genes that make the specific antigens

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the body needs to trigger its immune
response to specific pathogens.

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When injected into the human body,

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those genes instruct cells
in the body to make the antigens.

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This causes a stronger immune response,

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and prepares the body
for any future threats,

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and because the vaccine only includes
specific genetic material,

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it doesn't contain any other ingredients
from the rest of the pathogen

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that could develop into the disease
and harm the patient.

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If these vaccines become a success,

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we might be able to build
more effective treatments

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for invasive pathogens in years to come.

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Just like Edward Jenner's
amazing discovery

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spurred on modern medicine
all those decades ago,

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continuing the development of vaccines

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might even allow us
to treat diseases like HIV,

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malaria,

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or Ebola, one day.