Embedded Files
Interactions: The following processes cause multiple interactions between viruses and humans and among humans.
2. The stages of infectious disease progression
Epidemiologists identify four main stages that people go through during an infectious disease outbreak: susceptible, exposed, infectious, and recovery.
Using COVID-19 as an example, susceptible people have not been infected by the SARS-CoV-2 virus or have not developed immunity, so they are at risk of infection. Once susceptible people are infected, they become exposed to the virus. The virus begins to reproduce, i.e., incubate, in the body. Exposed individuals don't show any symptoms during the incubation period, but they can infect others as the incubation period comes to an end. The individuals are considered infectious when symptoms like fever, cough, shortness of breath, and fatigue appear. The infectious people will go through the disease period, after which they either recover or, unfortunately, may die. If they recover, they usually become immune to the virus strain they were infected with (Figure 23).
Figure 23: SEIR (Susceptible-Exposed-Infectious-Recovered) model that describes the interactions among people during epidemics
Decentralization: As we can see, many interactions occur among people and between people and viruses, and these interactions are carried out by individual people or viruses. Therefore, all interactions are decentralized. Note, that even though governments may implement lockdowns or travel restrictions, these policies are still carried out by each individual.
Stochasticity: Variations exist in many parts of this system, causing uncertainties, such as the likelihood of someone getting infected, the chances of recovery, or the risk of death. Variations also exist on whether and how people comply with government policies, Viruses may mutate randomly, generating new strains.
Emergence: The increase and decrease of COVID-19 cases are emergent phenomena of this system, typically depicted as an epidemic curve (Figure 24). A steep, narrow curve represents a rapid epidemic over a short time, which puts high pressure on public health facilities. Lockdowns, travel restrictions, PPE, social distancing, and vaccination can help "flatten the curve" to decrease daily cases but may prolong the duration of the epidemic.
Figure 24: An epidemic curve shows the increase and decrease in disease cases over time. Its shape emerges from the interactions among the people and the disease pathogen. The graphic above shows the epidemic curves of COVID-19 in Italy, Germany, and the United Kingdom from February 2020 to July 2020.
Page updated
Google Sites
Report abuse