Despite advances in technology, one of the biggest threats to human survival comes from primitive(原始） life forms.
Infectious diseases are caused by microorganisms, such as bacteria, viruses and parasites.
These organisms have no access to education or technology, but they present （带来）a huge challenge to humanity.
That's because they are adaptable.(可修改的，适应的）
Just as adaptability has helped humans survive, it makes it more difficult to control infectious diseases.
As condition's change, the organisms which cause these diseases adapt and become stronger.
There are several kinds of infectious diseases.
Bacterial infections, such as pneumonia(肺炎）, are caused by bacteria.
These infections can be treated by antibiotics(抗生素）, which kill the bacteria.
Viral (病毒的）infectious are caused by viruses, such as the influenza（流感） virus.
These can sometimes be prevented by vaccines(疫苗）, such as the flu vaccine.（流感疫苗）
An example of a parasitic (寄生虫d）infection is malaria（疟疾）.
Malaria is caused by parasites transmitted to humans by infected mosquitoes.
Once inside the body, the parasites（寄生虫） can be difficult to kill or control.
And of course these parasites are also adaptable.
How serious are infectious diseases?
Consider what happened in 1918, the influenza pandemic(流行病，侧重覆盖范围广）.
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 to be the most deadly epidemic（传染病,侧重传染性大 ) 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(死亡率） 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 (流行病）was that it was most deadly for people aged 20 to 40, rather than the elderly and young children.
