Despite advances in technology, one of the biggest threats to human survival comes from primitive（原始的） life forms.
Infectious diseases are caused by micro-organisms, such as bacteria, viruses, and parasites.
These organisms have no access to education 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 organism which causes these diseases to 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.
Antibiotics attack and kill the bacteria
Viral infections are caused by viruses, such as the influenza virus.
These can sometimes be prevented by vaccines, such as flu vaccines.
An example of parasitic（寄生的） infection is malaria（疟疾）.
Malaria is caused by parasites（寄生生物） 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.
How serious are infectious diseases?
Consider what happened in 1918, the influenza pandemic（流感大流行）.
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 two and a half percent compared to a point one percent in previous influenza epidemics. （它的死亡率是百分之二点五，而在以前的流感流行中只有百分之一。）
It struck quickly and without mercy, sometimes killing people within hours.
Schools and business 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.
Now, a century later, we have flu vaccines that have reduced the severity of the epidemics. （现在，一个世纪后，我们有了流感疫苗，可以减轻流行病的严重程度。）
These vaccines need to be updated every year because the virus changes and new strains emerge.
What researchers fear is that a new strain of the virus will develop and cause another pandemic.
A new strain of a virus is one that has mutated from the original.
Once an outbreak occurs, transmission of the disease will be even faster than before.
An unintended consequence of our improved transportation system will be the speed at which an epidemic can spread.
Experts predict that public health systems will be overwhelmed and unable to react quickly enough.
So the best defense will be to prevent an outbreak from occurring.
This will require global cooperation and the sharing of data.
It may also require changes in how humans interact with animals.
Animals, such as birds and pigs, carry viruses that may mutate（突变） into a strain that can infect humans.
Such a new strain could lead to another pandemic well before a new vaccine could be developed.
Since diseases and epidemics don’t stop at national borders, this is a global issue requiring global action.
The threat of a global epidemic is another example of why we must learn to live together.
