There are three main categories of factors which can affect disease transmission.

• Pathogen factors

• Environmental factors

• Host factors

These factors may be influenced by humans intentionally through things such as quarantine and vaccination, or it may be unintentional such as agriculture, urbanisation or transport. What are the pathogen factors affecting the spread of disease?

1. Mechanism of transmission: This may restrict particular diseases to certain environments, for example, Malaria is only present where mosquitoes thrive as it is a mosquito-borne vector disease.

2. Pathogen infectivity: The transmission of a pathogen is dependent on its ability to spread from one host to another, i.e. how easily an infection can be established in a susceptible host.

3. Incubation period: The period between infection and the presentation of symptoms may increase transmission, as individuals don't feel sick but can still spread the disease.

4. Asymptomatic carriers: These are individuals who carry the disease but don't present with any symptoms, and so, similarly to people in the incubation period of a disease, may spread the disease more as a consequence of not showing symptoms.

5. Host persistence: The persistence of a pathogen within its definitive host or intermediate host affects the spread of disease.

What are the environmental factors affecting the spread of disease?

1. Efficiency of infrastructure: Things such as water supply, roads and sewage symptoms, and the general level of hygiene and degree to which a community has well-established, efficient infrastructure can impact the transmission of disease.

2. Climate change: The changing climate in terms of rising water/air temperatures means that certain diseases may have an altered level of transmission, for example the increase in temperature and change to rainfall effect the spread of diseases such as Malaria, which while usually confined to tropical areas now has the opportunity to spread to more urban areas.

3. Globalisation: The increasing rate of globalisation, meaning a greater level of interconnectedness between other economies with our own in terms of trade, the movement of goods and services and the movement of people between countries impacts the spread of disease.

4. Natural disasters: Extreme climatic events may promote the spread of a disease, such as floods or tornadoes; in these situations, displacements of a population and breakdown of usual sanitation, food, health infrastructure are major contributing factors.

What are the host factors affecting the spread of disease?

1. Age of the host: Elderly individuals and young children have either weakened or undeveloped immune systems, and so they are more likely to contract a disease and spread it to other susceptible hosts.

2. Immunocompromisation: Individuals who are immunocompromised are more susceptible to contracting a disease, and the symptoms will usually be more severe.

3. Decision to get vaccinated: The choice to be immunised for diseases is a major host factor influencing the spread of disease; if individuals do choose to be vaccinated, this can create herd immunity which protects other non-immunised individuals in the population. If individuals do not choose to be vaccinated, there are more pathways for a disease to be transmitted and more people are likely to contract said disease.

4. Mode of transmission: Different infections are more likely to spread in certain groups within a population; for example, a group of people who share needles are more likely to contract hepatitis C and HIV.

5. Travelling: When carriers of infected individuals travel into other populations with no previous exposure, this allows for the spread of a disease over a large geogrpahical area, and it has devastating effects due to the population having no immunity.