Despite advances in technology, one of the biggest threats to human survival comes from primitive life forms.
Infectious deseases are caused by microorganisms, such as bacteria, viruses and parasites.
These organisms have no access to eduction or technology, but they present a huge challenge to humanity.
That's because they are adaptable.
Just as adaptability has helped humans survive, it makes it more difficult to control infectious diseases.
As conditions change，the organisms which cause these diseases adapt and become stronger.
There are several kinds of infectious diseases.
Bacterial infections, such as pneumonia, are caused by bacteria.
These infections can be treated by antibiotics, which kill the bacteria.
Viral infections are cause by viruses, such as the influenza virus.
These can sometimes be prevented by vaccines, such as the flu vaccine.
An example of a parasitic infection is malaria.
Malaria is caused by parasites transmitted to humans by infected mosquitoes.
Once inside the body, the parasites can be difficult to kill or control.
And of course these parasites are also adaptable.
（Antibiotics attack and kill bacteria.）
How serious are infectious diseases?
Consider what happened in 1918, the influenza pandemic.
It started in the fall of 1918 with what seemed to be something no more serious than the common cold.
In the next two years however, it infected a fifth of the world's population.
In fact, it turned out to be the most deadly epidemic in history, killing between 20 and 40 million people.
The influenza virus that caused it was much stronger than previous influenza viruses.
Its mortality rate was 2.5% compared to 0.1% in previous influenza epidemics.
It struck quickly and without mercy，sometimes killing people within hours.
Schools and businesses closed in many places，and people wore masks to prevent transmission of the virus through the air.
What was unusual about this pandemic was that it was most deadly for people aged 20 to 40，rather than the elderly and young children.
（People wore masks to prevent transmission of the virus through the air.）
