Origins of Viruses/ Viral Transmission
Virus Origins: There is a lot of debate on this topic, but scientists have developed a few proposed hypotheses for the origin of viruses:
- The Progressive Hypothesis: Viruses originated from the evolution of mobile genetic elements that gained the ability to move between cells.
- The Regressive Hypothesis: Viruses evolved from more complex free-living organisms that lost genetic material as they adapted to a parasitic approach to reproduction.
- The Virus-First Hypothesis: Viruses existed before cells as self replicating units that eventually became more complex and may have led to the rise of the first cells. Also known as the coevolution hypothesis.
Types of Transmission:
- Droplet (sneezing/coughing)
- Vector (example: mosquitos are a vector for several diseases such as malaria and dengue fever)
- Airborne (inhaling) (examples: rhinovirus, influenza A virus)
- Waterborne (from infected water, such as cholera, botulism, dysentery)
- "Sit-and-Wait" (example: smallpox)
- Sexual (HIV, HPV, chlamydia)
- Fecal-oral (poliovirus, hepatits A virus)
is an article on routes for the spread of infectious disease.
Virulence Trade-off Hypothesis: More virulent pathogens grow more quickly within the host, and therefore overwhelm and kill the host faster. These viruses can still increase in frequency as long as the pathogen can transmit itself to a new host, whether it be before or after the host dies. However, a virus that kills the host too quickly may not be able to spread as much. Therefore, other types of diseases may out-compete it. A virulent virus is harmful to the host, takes less time to spread, and is less likely to spread. A less virulent virus is less harmful to host, but has more time to spread, and is more likely to spread. Therefore, viruses areconstantly changing due to their own characteristics, their interaction with the host, and the environment the host is in. Densely populated areas and air travel have made it easier for more virulent diseases to spread. For example the ebola virus is severely virulent but it rips through populations so quickly that it rarely spreads out of the original geographic region. On the other hand a virus such as influenza can spread throughout a population easier because it is less virulent and allows the host more time to come into connect with other potential hosts.
- Here is an article explaining how the flu virus may be mutating to increase transmissibility.
A cool simulation of virulence is the smartphone app Plague Inc. It is super fun and lets you engineer a disease with the goal of wiping out the human population. It appears to have been based on the online game Pandemic II, another game in which you engineer an apocalyptic disease. The strategy involves a balance between virulence and infectivity -- you can't let your disease become too lethal, too fast or else you risk people reacting to the illness and finding a cure. Furthermore, if your disease is too deadly, it becomes difficult for it to spread before killing its host. This level of strategy mirrors the difficulty a virus could have in surviving and spreading in real life.
It is important to understand why viruses are so harmful and why their evolution is so quick. A virus that has a high chance of surviving must be able to transmit from host to host and must be able to replicate quickly. There are balances and intermediates to these criteria of surviving. For example, if the virus replicates too quickly, it can kill the host before the host can transmit the virus. The ideal virus will weaken the host, but not kill it so it can still transmit the disease. Urbanization has made transmission easier for viruses to survive, replicate, and evolve. Evolution happens by the many mutations that happen when replicating the genome.
Infecting a Host
It is important to understand the basics of how viruses infect host (ex: bacteria) They do so by incorporating their DNA into the host's DNA. They can then go into one of two different phases: the Lytic Cycle or the Lysogenic cycle.
Lytic - This is where the phage DNA circularizes to form its own plasmid and uses the proteins for transcription and translation in the host cell to make new phages. This continues until the host cell is lysed and releases the newly formed phages.
Lysogenic - once the phage DNA is incorporated into the cell it then binds and becomes part of the hosts DNA. It inserts itself into the hosts plasmid DNA and can then lay "dormant" for as many cell cycles as needed. This happens based on factors that would not favor the production of new viruses. This can be part of the genome for many generations.The important thing about this cycle is that it can then propagate its progeny.
Here is a video that explains each type of viral propagation/replication.
Diagram to help you understand better:
If a highly pathogenic form of a virus from a different animal host and is able to infect a human, and it is able to recombine DNA with the human strain of the virus, this can lead to a very pathogenic new virus. The animal strain is able to combine its pathogenesis with the ability to successfully infect humans from the human strain, thus creating an incredibly virulent new strain.
Example: evolution of bird flu (can be very virulent strains)
Viral Reassortment - Occurs when genetic material from 2 or more different strains of a virus combine within a single host producing often dangerous new strains. A common example is the bird flu reassorting with the human influenza, enabling the bird flu to infect the human genome.
- There is usually a trade-off between the virulence of a virus strain and it's ability to transmit itself effectively. The more virulent a strain is, the less likely it is able to infect another organism before it kills it's current host. However, viral reassortment can produce strains that are both very virulent and capable of transmitting between hosts; leading to a deadly combination.
- This can occur when a strain from a different species reassorts with a strain that is already infectious to a human host, producing a completely new strain.
- A common example of this is the seasonal influenza virus, which explains why people who get the flu vaccine, may still be succeptible to new strains.
- Here is a video that shows the process of viral reassortment
- example: H1N1 virus. This virus that infected and harmed many people a few years ago came from the co-infection of a pig strain and a human strain.
Videos, Articles, and Diagrams
- A good website describing the basics of viral evolution can be found here.
- Vaccine Vigilance. This article is directly related to our lecture over the evolution of medicine and how the evolution of viruses and other pathogens directly causes the evolution of vaccinations and medicine we use. This article discusses the option of creating a better vaccine for a "more virulent malaria."
- Predicting HIV drug response: Click Here!!
- How the flu evolves so quickly, and how a vaccine could be developed that would last for longer than a year.
- Here is video the explains the evolution of viruses, as well as host invasion, and viral mutations.
- This video talks about the evolution of virus mutations as well as the transmission of viruses, their propagation, and their resilience.
- This article explains viral evolution, and it displays a good example of HIV and influenza viruses.
- Here is a great video giving the example of H1N1 virus transformation.
- This video to gives you an informal, but informative look into what a virus is and how it behaves.
- Viruses such as Ebola do not necessarily take a direct path in infecting hosts. The virus is normally transmitted thorough multiple vectors of differing species before affecting humans. Once a human is affected, the body has little time to receive treatment before the victim is compromised. Usually it consists of necrosis and bleeding out through the eyes, ears, and any other bodily opening.
- A very interesting view on viral evolution.
- This article explains viral evolution and details that all RNA viruses have about the same mutation rate, which leads to much change.
- Here is a video about the flu of 1918 and how and why it was so deadly.
- This page discusses the 1918 flu
- This article talks about the idea of a universal vaccine that can protect individuals from multiple strains of influenza. The way this vaccine would work is by provoking the body to create antibodies that target the stems of surface proteins on viruses instead of the surface proteins themselves, as the stem proteins change very little compared to the surface proteins. Scientists have been able to create antibodies that attach to the stems and protect against multiple strains in mice.
- http://www.learner.org/courses/biology/textbook/hiv/hiv_4.html This website shows the life cycle of HIV.