Disease
An disease is any condition that meddles with the ordinary working of the cells, tissues, organs and the entire body frameworks. Sicknesses of poultry have numerous causes and include:

  1. Deficiencies of essential nutrients such as vitamins, minerals or other nutrients.
  2. The consumption of toxic substances such as poisons.
  3. Physical damage e.g. environmental extremes and injury.
  4. Internal and external parasite infestations such as lice and worms.
  5. Infectious disease caused by micro-organisms such as bacteria and viruses.
 Diseases that outcome from supplement lacks, utilization of lethal substances and physical harm are alluded to as non-irresistible infections. These maladies can't be passed from feathered creature to fledgling and individuals from the run must share a typical affair for people to get these non-irresistible illnesses. In the most extensive sense, irresistible maladies are caused by microorganisms that incorporate parasites, growths, protozoa, microscopic organisms, mycoplasmas, chlamydia and infections. These infections are regularly likewise called infectious maladies implying that they can be passed starting with one fowl then onto the next either specifically or by implication.

Coordinate or Direct transmission happens when one ailing winged animal passes the reason for the infection by means of direct contact to a defenseless sound feathered creature. Such section might be flat transmission (starting with one feathered creature then onto the next) or vertical transmission (from parent to posterity) through the egg or sperm either inside the egg or on the shell. Aberrant or Indirect transmission happens when the causal living being is passed starting with one winged creature then onto the next by means of a middle host, for example, creepy crawlies, worms, snails or slugs, wild fowls or creatures or some other item, for example, hardware, nourishment or water, vehicles, individuals, respiratory beads, litter or dung.

Causes of infectious disease
Organisms and microorganisms that can possibly cause hurt, for example, sickness in creatures, are called pathogens or ailment vectors. There are a wide range of kinds of pathogens that might be exchanged starting with one fowl then onto the next or starting with one rush then onto the next by a wide range of methods. These pathogen types include:

  1. Viruses
  2. Bacteria
  3. Fungi
  4. Protozoa
  5. Internal parasites
  6. External parasites

Viruses
Viruses are the smallest pathogens and can only be seen through an electron microscope. Viruses consist of an outer layer/s surrounding special protein material similar to the genetic material of the cells they invade. They can multiply and do harm only when inside the animal cell and if they invade and damage enough cells, the animal can show signs of that infection.
Antibiotics and other medications as a rule do not affect viruses and, as a consequence, there are very few medications that can treat diseases caused by viruses, although there are times when a drug may be used to control secondary infections. The best way to manage diseases caused by viruses is by quarantine and good hygiene to lower the challenge, and vaccination to maximise the birds’ immunity to future challenges. Some have the ability to survive for very long periods of time in the bird dander and feather debris, litter and manure, insects and rodents.
Bacteria
Bacteria are single cell organisms with a nucleus and multiply by simple fission, which means that one divides into two, and some can do this very quickly inside the host or in a suitable environment. Some are very fragile and do not survive long outside of the host while others may survive for long periods even in a harsh environment. Many have the ability to turn into spores by forming a very tough wall that protects them from most of the materials used to kill them. These types of bacteria are much more susceptible to these compounds when not in the spore form.
Bacteria may be described as being gram positive or gram negative. This characteristic is to do with differences in their cell walls that affects their staining for viewing under the microscope. Whether they are one or the other also influences their response to certain chemicals, including disinfectants.
Different types of bacteria harm the birds in two predominant ways:
  1. Those that attack and damage the birds’ cells or spaces between the cells.
  2. Those that produce toxins or poisons that harm the birds.
There are several antibiotics and other drugs that are effective against different bacteria. However, quarantine and good hygiene that lower the numbers to be targeted by the drugs are the important first lines of defense against these organisms.
Chlamydia
Chlamydia are a little larger than viruses. They live inside the cells they infect particularly in the cells of the respiratory system. They can be treated with antibiotics.
Mycoplasmas
These are single cell organisms slightly larger than chlamydia. They have a cell wall and nucleus. The most commonly known disease caused by this organism is Mycoplasmosis or Chronic Respiratory Disease (CRD) caused by Mycoplasma gallisepticum. Diseases caused by Mycoplasma organisms respond to some antibiotics. These organisms do not survive long outside of the host and good quarantine and hygiene procedures coupled with a suitable house de-population period will provide good control.
Fungi
Fungi are organisms larger than bacteria and are considered to be members of the plant kingdom. They multiply by forming spores that are released and enter the local environment. When conditions are satisfactory the spores start to grow to repeat the cycle.
Fungi harm the birds in two ways:
  1. By being taken into the body e.g. in the respiratory system where they start to grow.
  2. By producing toxins or poisons e.g. in the food. When the birds consume the contaminated food the toxin affects them. A good example of this type of damage is aflatoxin produced by certain moulds or fungi that commonly grow in peanut meal and some litter materials. Moulds or fungi are resistant to nearly all antibiotics.
Protozoa
Protozoa are single cell organisms larger than bacteria. Protozoa have a complex reproduction system that, in many cases, allows them to multiply into extremely high numbers very quickly. A good example of protozoan diseases is coccidiosis of poultry.
Protozoa generally harm the birds by destroying tissue. A number of chemicals have been developed that can be used to treat birds infected by the different protozoans. Others have been developed that interfere with the protozoan life cycle and may be used as preventive treatments while the birds develop a natural immunity. These preventive drugs are often referred to as coccidiastats.
Internal parasites
Parasites are organisms that live off the host. Internal parasites in poultry are multi-celled organisms that live inside the bird usually located in specific organs. Most internal parasites, and particularly those found in Australia, are visible to the naked eye.
While there are many different internal parasites found in poultry, only three are likely to cause harm. These are:
  • Large round worms
  • Caecal worms
  • Tape worms
External parasites
These parasites live outside of the bird. Some spend all of their life on the bird while others spend only some time on the birds. Some cause harm by irritating the bird while others are bloodsuckers that, in sufficient quantity, will cause anaemia. Some of the bloodsuckers often carry organisms called spirochaetes that they inject into the bird while feeding. The spirochaetes may cause harm and tick fever is a good example that can kill many birds.

                                                                                                >>>>>>>>