Infectious diseases have to reach and thrive in living organisms. In this lesson, we will figure out what are the agents that cause infectious disease. 

Based on the diseases you are aware of and that you saw in the phenomenon video, make a list of as many things that you think cause disease. 

The vast majority of infectious diseases are caused by microorganisms, called "microbes." However, only about 1% of microbes cause disease! We call these microbes who cause disease, pathogens.

You are probably familiar with many of these pathogens. Let's look at the ones we saw in the phenomenon video first. Read the following and then answer the questions below. 

White Nose Syndrome:

White-nose syndrome (WNS) is a disease that affects hibernating bats and is caused by a fungus, Pseudogymnoascus destructans, or Pd for short. Sometimes Pd looks like a white fuzz on bats’ faces, which is how the disease got its name. Pd grows in cold, dark and damp places. It attacks the bare skin of bats while they’re hibernating in a relatively inactive state. As it grows, Pd causes changes in bats that make them become active more than usual and burn up fat they need to survive the winter. Bats with white-nose syndrome may do strange things like fly outside in the daytime in the winter. 

Fire blight on apple tree:

Fire blight is a common and very destructive bacterial disease of apples and pears. The disease is caused by the bacterium Erwinia amylovora. On apples and pears, the disease can kill blossoms, fruit, shoots, twigs, branches and entire trees. While young trees can be killed in a single season, older trees can survive several years, even with continuous dieback.

Brain worm in Moose: 

Brain worm is the term commonly applied to the parasitic nematode (round worm), Parelaphostrongylus tenuis (P. tenuis). White-tailed deer are the normal host for this parasite. Most of the time, they are not affected by the parasite. However, other species such as moose, mule deer, reindeer/caribou, sheep, goats, alpacas, and llamas are abnormal hosts and can develop disease or die if infected.

Sea Star Wasting Syndrome: (SSWS/SSWD)

In December 2020, sunflower sea stars were placed on the ICUN's red list of threatened species. There has been a 90% decline in this species due to warming oceans an an epidemic called "Sea Star Wasting Syndrome." Scientists from The Nature Conservancy and Oregon State University, who led the listing effort, estimate that 90.6% of the sunflower sea star population is now lost from the outbreak, with as many as 5.75 billion dead from the disease.  Symptoms of the disease include a deflated appearance, white lesions and twisted arms, followed by softening tissue, loss of arms, and death. The disease progresses rapidly, often killing its victims within a matter of days. In an effort to identify the potential pathogen and conditions responsible for SSWD, scientists and their partners collected extensive survey data and tissue samples, but the cause remains a mystery that several research institutions are working hard to unlock.

We have looked at a few examples of pathogens. What are the agents who are causing disease in plants and animals?

Bacteria:

Bacteria are microscopic, single-celled organisms that have no nucleus and a cell wall made of peptidoglycan. Bacteria are the direct descendants of the first organisms that lived on Earth, with fossil evidence going back about 3.5 billion years.
Most bacteria are much smaller than our own cells, though a few are much larger and some are as small as viruses. They usually do not have any membrane-wrapped organelles (e.g., nucleus, mitochondria, endoplasmic reticulum), but they do have an outer membrane. Most bacteria are also surrounded by at least one layer of cell wall.
Bacteria are a huge and diverse group. Its members have many shapes, sizes, and functions, and they live in just about every environment on the planet.

Viruses: 

Viruses are microscopic particles made of nucleic acids, proteins, and sometimes lipids. Viruses can’t reproduce on their own. Instead, they reproduce by infecting other cells and hijacking their host’s cellular machinery. 
Since the ability to reproduce is often listed as a requirement for life, some consider viruses to be non-living. Regardless, viruses are an important part of all ecosystems, including the human body.
In our bodies, viruses infect not only our cells, but also other microbes that live in our bodies. Viruses that infect bacteria are called bacteriophage. 
Think of a virus as a tiny package jacketed in a protein covering. Inside is either DNA or RNA. Each molecule serves as an instruction book. Its genetic information provides instructions that tell a cell what to make and when to make it.

Fungus: 

There are millions of fungal species, but only a few hundred of them can make people sick. Molds, yeasts, and mushrooms are all types of fungi.
Fungi can cause many different types of illnesses, including:

• Asthma or allergies. Learn more about mold and how it can affect your health.
• Rashes or infections on the skin and nails
• Lung infections (pneumonia), with symptoms similar to the flu or tuberculosis
• Bloodstream infections
• Meningitis

Anyone can get a fungal infection. Fungi are common in the environment, and organisms breathe in or come in contact with fungal spores every day without getting sick. However, in organisms with weak immune systems, these fungi are more likely to cause an infection.

Parasites: 

Parasites are living things that use other living things - like your body - for food and a place to live. You can get them from contaminated food or water, a bug bite, or other types of direct contact. Some parasitic diseases are easily treated and some are not.

Parasites range in size from tiny, one-celled organisms called protozoa to worms that can be seen with the naked eye. Some parasitic diseases occur in the United States. Contaminated water supplies can lead to Giardia infections. Cats can transmit toxoplasmosis, which is dangerous for pregnant women. Others, like malaria, are common in other parts of the world.

Prion: 

Prion diseases comprise several conditions. A prion is a type of protein that can trigger normal proteins in the brain to fold abnormally. Prion diseases can affect both humans and animals and are sometimes spread to humans by infected meat products. 

Prion diseases occur when normal prion protein, found on the surface of many cells, becomes abnormal and clump in the brain, causing brain damage. This abnormal accumulation of protein in the brain can cause memory impairment, personality changes, and difficulties with movement.

Prions as a disease causing agent were only discovered in the late 20th century, with an American Biologist receiving the Nobel prize for his discovery in 1997. Experts still don't know a lot about prion diseases, but unfortunately, these disorders are generally fatal.

It is important to remember that:

• A pathogen is a micro-organism that has the potential to cause disease.
• An infection is the invasion and multiplication of pathogenic microbes in an individual or population.
• Disease is when the infection causes damage to the individual’s vital functions or systems.
• An infection does not always result in disease!