Notes

 

Infectious Disease


Infectious diseases are caused by pathogens; any agent that causes disease.  Virus is a cause of some common infectious diseases.  A tiny disease-causing particle that invades a cell to multiple is called a virus.  A virus is not a living thing.  It is so small it can only be seen with a microscope.  Some viruses are harmless, while other viruses cause diseases.

EXAMPLES OF VIRUSES

·      Flu and Cold

·      Herpes

·      AIDS

·      Chickenpox

·      Mumps

·      Measles

Vaccines can prevent some viruses.  A vaccine is a medicine that contains killed or weakened pathogens and protects you against a particular disease.  They stimulate your immune system to produce antibodies.  An antibody is a chemical that your body produces to fight invading pathogens.

Another cause of disease is bacteria.  Bacteria are living things.  They are more complex than viruses.  They are small, single celled organisms without a nucleus.  Most bacteria can not live on their own, but are found everywhere.  Most are harmless and are helpful to humans (such as the kind that live in your intestines).  Humans can’t live without bacteria.  But some bacteria can cause serious illness.

SOME DISEASE IN HUMANS CAUSED BY BACTERIA:

·      Whooping Cough

·      Cholera

·      Anthrax

·      Tooth decay

·      Some kinds of Food Poisoning

Bacteria can be controlled by washing your hands and by keeping your kitchen clean.  It can also be controlled by not sharing drinking glasses, eating utensils and if the water supply has harmful bacteria in it, the water must be boiled or treated with chemicals.  Most bacterial diseases can be treated with antibiotics.  An antibiotic is a medicine used to stop the growth of or kill the bacteria.

WAYS THAT VIRUSES SPREAD

Direct Contact, touching a person

Indirect Contact, sneezing, objects (drinking glasses, insects)

Protozoa and fungi cause some diseases.  Protozoa are single celled organisms that have a nucleus.  They are more complex than bacteria.  Most protozoa are harmless but some cause illness.  Some protozoa are parasites.  A parasite is an organism that gets it food from another organism, called a host.  Protozoa can be found in water and carried by insects.  Antibiotics can treat protozoa and other parasites.  Fungi are complex organisms that cannot make their own food.  They break down other substances and absorb their nutrients.  Fungi are everywhere.  Most fungi are harmless, but some cause diseases.  Person to person contact spreads some fungi.  Spores in the air, water or soil spread others.  Athlete’s foot is an example of fungi.  To get rid of fungal infections, such as toenail fungus, you take medicine by mouth.


IMMUNITY AND DISEASE 


FACTORS THAT CAUSE DISEASES


Diseases may be caused by one of the following factors, or by a combination of several of these factors.





The Body’s Defenses


Our bodies are surrounded by microorganisms trying to get into us through the nose, through cuts in our skin, or along with the food we eat. Controlling these microscopic invaders is as important to homeostasis as is regulating body temperature and chemistry.


The lines of defense can be either nonspecific or specific. A nonspecific line of defense keeps out any microorganism. It does not matter what particular invading microorganism it is. A specific line of defense attacks only a particular microorganism, one type at a time.


The first line of defense against infection is nonspecific. It consists of physical barriers that block the entry of microorganisms. The skin because it is made up of a continuous layer of flat, tough cells, it protects the body from invaders as long as it is undamaged. Fluids are also released at certain places on the body to keep out microorganisms. For example, mucus in the passages of the respiratory system, saliva in the mouth, and tears around the eyes all contain substances that kill microorganisms. The strong acid in the stomach is very effective in killing microorganisms in the food we eat.


A second line of defense is present when microorganisms get through our physical barriers. After a cut or scrape on the skin the injured area becomes warm, reddened, and perhaps swollen with pus. The events described are called inflammation. When the injury occurs, chemicals are released by the damaged tissues. They cause an increase in the blood flow to the site of the injury. Responding to the alarm, phagocytic white blood cells that attack invaders by ingesting them, arrive.


Vertebrates have evolved an immune system. The immune system knows who the “bad guys” are and goes after very specific invaders.  In 1796 English physician, Edward Jenner, noticed that people who worked with cows frequently caught cowpox, a mild disease similar to smallpox, but did not usually contract the deadly disease smallpox. He therefore intentionally infected an eight-year old boy with cowpox. After the boy recovered from cowpox, Jenner deliberately injected the boy with smallpox. The boy did not get sick!


We now know that the immune system defends our bodies against very specific invaders. Each invader—usually a bacterium or virus—has specific protein molecules, called antigens, attached to its surface. These molecules that are detected by the body’s immune system. When the immune system detects an antigen, it produces antibodies. Antibodies provide this defense by binding to the antigens. As it turns out, the cowpox and smallpox antigens are almost identical. After he was injected with cowpox, the boy’s immune system made antibodies against the cowpox antigen. Later, when he was injected with smallpox, the boy’s body was ready with a defense. The smallpox virus was destroyed with the help of the antibodies that the boy had made against cowpox antigens. This technique came to be known as vaccination.


The immune system also includes B cells, produced in the Bone marrow, and T cells, produced in the Thymus, two types of macrophages. Macrophages move to infected areas to engulf and digest invading microorganisms. B cells are the ones that respond to specific antigens by beginning to produce antibody proteins that will bind only with that antigen. As time goes on, the body contains many different types of B cells, each producing antibodies for one specific antigen. After having been invaded once by an antigen, some special cells, called memory B cells, that recognize that antigen remain in the body for the rest of your life. Because they are already present in the body, you rapidly start making antibodies when you encounter the same invading microorganisms again. That is why individuals usually do not get measles or chicken pox a second time. The immune system remembers the first exposure to the disease and is ready! This type of protection is called active immunity.


Antibodies are good at recognizing antigens on invaders only when the invaders are in the fluids in your body but not inside your cells. Many bacteria and all viruses quickly get inside body cells, where they begin to multiply. Antibodies cannot find the invaders once they are inside cells. One type of cell is called cytotoxic, or killer, T cells can recognize cells in the body that have been infected with invading microorganisms. This recognition occurs when an antigen present on the surface of an infected cell binds specifically with a receptor protein projecting from the cell membrane of a T cell. Then the killer T cells punch holes in the membranes of the infected cells, sometimes injecting poison into them. The infected cells are killed and the invaders in these cells are destroyed. Another important type of T cell acts as a helper. Helper T cells assist both B cells and killer T cells. Without helper T cells, the other members of the immune system family cannot do their job. Just how important helper T cells are is shown by the fact that they are the cells destroyed by the human immunodeficiency virus (HIV), which results in the disease called AIDS.


In passive immunity, a person is injected with a large quantity of the correct already formed antibodies to a particular antigen. These antibodies protect the body from a disease only for as long as the antibodies remain in the body. The body did not make the antibodies and, more important, does not “remember” how to make them. You therefore have protection for a limited time from passive immunity.

WHEN THINGS GO WRONG: DISEASES OF THE IMMUNE SYSTEM


The immune system helps maintain the internal dynamic equilibrium necessary for life. However, the immune system can become out of balance. It can be overactive or underactive, and in either case the body’s equilibrium is upset.









NONINFECTIOUS DISEASE 

Infectious Disease