• Infectious diseases are usually caused by microorganisms that invade the body and multiply.

• Invasion by most microorganisms begins when they adhere to a person's cells.  
• Adherence is a very specific process.
  

• Whether the microorganism remains near the invasion site or spreads to other sites
• depends on such factors as whether it produces toxins, enzymes, or other
  substances.

• Some microorganisms that invade the body produce toxins. Some illnesses are caused by toxins produced by microorganisms outside the body.  

• After invading the body, microorganisms must multiply to produce infection.
• After multiplication begins, one of three things can happen: the microorganisms can
  continue to multiply and overwhelm the body's defenses; a state of balance can be
  achieved, producing a chronic infection; or the body—with or without medical

• treatment—can destroy and eliminate the invading microorganism.                                                                                                       
• Many disease-causing microorganisms have properties that increase the severity of the diseases they cause (virulence) and help them resist the body's defense mechanisms. Some microorganisms have ways of blocking the body's defense mechanisms.  

An infectious disease is caused by one or more of the following:

·        viruses

·         bacteria

·         parasites

·         fungi

An infectious disease can spread in some or all of the following ways:

·                     Air-borne transmission—transmission of an infection via the inhalation of air-borne droplets containing the organism, which may be present in the immediate environment as a result of an infected person having coughed or sneezed.

· Blood-borne transmission—transmission of an infection via contact with infected blood, such as through blood transfusions or when sharing hypodermic needles  

· Direct skin contact—transmission of an infection via contact with the skin of an infected person  

· Insect-borne transmission—transmission of an infection via insects, such as mosquitoes, which draw blood from an infected person and then bite a healthy person  

·  Food-borne transmission—transmission of an infection via the consumption of contaminated food  

·    Water-borne transmission—transmission of an infection via contact with contaminated water  

·   Sexual transmission—transmission of an infection via sexual contact, including intercourse  

·    Other mechanisms that can transmit a disease

http://uuhsc.utah.edu/healthinfo/adult/infectious/immune.htm

In the treatment of men and animals infected with an infectious disease, it is costumary to use medicine. The particular medicine has to act against the disease generator, without harming the patient. To that aim, an effort is being made to develop drugs that will operate in specific locations in the body, in order to minimize the drugs' negative side effects.

In the 20^th century, we witnessed major breakthroughs in the discovery and development of new drugs against bacteria. Scientists discovered a group of drugs called antibiotics, considerably improving our ability to specifically hurt those pathogens responsible for infectious diseases.

Artificial compounds or substances created in one organism and affecting another.

The difference between the various antibiotics lies in the manner by which they harm the bacteria.

Although antibiotic substances have specific activities, some antibiotic medicines may cause certain types of damage in man. In any treatment, the physician must consider the harm caused buy the disease versus that possible from the treatment.

To expand your knowledge of antibiotics, go to the following website:....