In the field of disease, pathogens are organisms found at the crossroads between species and their way of surviving and reproducing. At these specific crossroads, the actions of one organism hurt the other. The pathogenic organism is the one inflicting disease upon the victim.
The mechanism of transmission for pathogens can include carriers and disease reservoirs. Carriers are infected with the pathogen but do not suffer from disease as a result of infection. Sometimes, this state can make them better at spreading the pathogen to others who might be more susceptible. Disease transmission can be effective when many carriers act as disease reservoirs in a population. They carry a pathogen that does not affect them negatively, is perhaps dormant, but could still be spread to others. People with weak immune systems, the very young and the very old may suffer from disease. An example of a disease with large reservoirs in certain clusters around the world is tuberculosis.
In the case of malaria for example, the parasite-caused disease is transmitted to humans via mosquitoes. They are vectors of the malarial parasite. This means they carry around and move the pathogen and enable infection of humans, without which this wouldn’t happen.
Where disease is present constantly over time, it is said to be endemic, for example as tuberculosis is endemic to parts of India. When an otherwise absent or infrequent disease surges in cases in a population, it creates an epidemic. If an epidemic spreads internationally, then it becomes a pandemic.
One way of controlling the prevalence of a disease is through large scale vaccination. Vaccination uses a harmless part of a pathogen, such as a viral surface protein, to cause the immune response to act as if the infection was by the real pathogen. This teaches the immune system to recognise the pathogen for the future, without actually having to go through the disease. The recognition part of a pathogen, such as the protein mentioned, is called an antigen. It’s a unique molecular entity that IDs an organism and the immune response is sensitive to. The molecule made in response to an antigen is an antibody which binds it very specifically, thus recognising it.
As a result of the arms race between pathogens and the immune system of their hosts, pathogens such as viruses have developed multiple possible variations in their antigens. These are known as antigenic types and some viruses such as rhinovirus can have hundreds of them.
If the pathogen is a bacterium, there are several different classes of chemicals that can kill them. These are called antibiotics. We use them because they can target bacteria while leaving out our eukaryotic cells. For example, if they target the production of bacterial cell walls, our cells won’t be affected as they do not have cell walls (just plasma membranes).
Because antibiotics act on the biological pathways of bacteria, these can get around the antibiotic by finding a different way of carrying out their functions. Basically, they can out-evolve the drug. This happens if antibiotics are not used for long enough to attack all bacteria in an infection (thus leaving alive the strongest, and letting those grow again) or if an antibiotic has been used heavily against a pathogen in a population. These scenarios lead to bacterial resistance. If a gene that confers resistance to an antibiotic ends up being selected for strongly, many bacteria will start having it, as they can be shared amongst them easily, not just passed down. The antibiotic will no longer work.
Many bacteria cause disease because they release substances that are toxic to their host. This is the case with cholera which produces toxins that lead to the symptoms of water loss through severe diarrhoea.
Alongside pathogens, the human body can be, and indeed depends on, other organisms it hosts. For example, the rich intestinal flora contributes to digestion by fermenting food in its later stages, and existing flora elsewhere acts as a defence against some invading pathogens by preventing or limiting their expansion.