An infectious disease is a disease that is caused by the invasion of a host by pathogens（病原体）whose activities harm the host's tissues and can be spread to other individuals.
Microorganisms（微生物）capable of causing disease are called pathogens. A true pathogen is an infectious agent that causes disease in any possible host. The terms "infection" and "disease" are quite different. In order to cause disease, pathogens must be able to enter the host body, stick to specific host cells, invade and multiply and do damage to host tissues.
Pathogens may be spread through either direct or indirect contact. Direct contact occurs when an individual is infected by contact with the infection source. It also includes taking in the infectious droplets released by sneezing or coughing. Indirect contact occurs when a pathogen can survive the environment outside its host for a long period of time. So lifeless objects that are polluted by direct contact with the infection source may be the indirect contact for easily infected group.
Public health measures typically involve killing the pathogen from its source or from its route of spread. Those measures include ensuring a safe water supply,effectively treating waste water,and initiating animal control and vaccination（疫苗）programs,etc. Personally,the first line of defense is to keep pathogen at bay by following good personal hygiene（卫生）habits. Prevent infection before it begins and avoid spreading it to others with some easy measures, such as washing hands, getting vaccinated and so on.
Man never stops fighting against all kinds of infectious diseases. But in the past two decades at least a dozen "new" disease have been identified,and traditional diseases that appeared to be "on their way out" are re-emerging. Globally,infectious diseases remain the leading cause of death. Clearly,the battle has not been won. Nevertheless, it is increasingly difficult for most of us to deny the claims of science. We are continually presented with great amounts of relevant scientific and medical knowledge, which encourages us to take more responsibility for our own health.



What does paragraph 2 focus on？ ______

A. Origin of true pathogens.
B. Nature of infectious diseases.
C. Strategies for avoiding tissue damage.
D. Differences between infection and disease.

What is likely to cause disease spread through indirect contact？ ______

A. Getting bitten by an infected dog.
B. Playing toys a sick child played with.
C. Breathing in tiny drops from sneezing
D. Shaking hands with an infected person.

Which of the following can best explain the underlined phrase "at bay" in paragraph 4？ ______

A. At risk.
B. Off course.
C. Under control.
D. Within reach.

What does the last paragraph imply？ ______

A. Man is at a loss about infectious diseases.
B. Traditional infectious diseases are dying out.
C. Science counts in defeating infectious diseases.
D. The battle against infectious diseases is in vain.