Infection and disease

Infection, often the first step, occurs when bacteria, viruses or other microbes that cause disease enter our body and begin to multiply. As long as a person is immune, there is no disease. Disease occurs when the cells in our body are damaged — as a result of the infection — and signs and symptoms of an illness appear.

EPIDEMIC is a disease that affects a large number of people within a community, population, or region. An epidemic is often localized to a region, but the number of those infected in that region is significantly higher than normal.

Examples – diseases such as Leptospirosis is a bacterial zoonosis transmitted via contact with rodents, domestic animals and contaminated water. Meningitis is an inflammation of the meninges, the lining surrounding the brain and the spinal cord. Neisseria meningitidis can cause large-scale epidemics.

PANDEMIC is an epidemic that is spread over multiple countries or continents. COVID-19 was limited to Wuhan, China, it was an epidemic. The geographical spread turned it into a pandemic.

ENDEMIC is something that is limited to a particular people or country. Endemics are a constant presence in a specific location. Malaria is endemic to parts of Africa.

An outbreak is a greater-than-anticipated increase in the number of endemic cases. It can also be a single case in a new area. If it’s not quickly controlled, an outbreak can become an epidemic.

Sporadic diseases are seen only occasionally, usually without geographic concentration, occurring  in a scattered, isolated, or seemingly random way. Examples of sporadic diseases include tetanus and rabies etc.

Prosodemic A disease transmitted directly from person to person. They can be passed from person to person through body secretions, insects or other means. Examples are SARS, influenza, the common cold, tuberculosis (TB)

Zoonotic diseases: Diseases which are spread from animals to humans, such as avian influenza, are known as zoonotic diseases. 

Epizootic: An epidemic outbreak of disease in an animal population and often it may extend to humans. For example, Rift Valley fever (RVF). It generally refers to outbreaks of disease which cause serious economic or public health issues and are of major importance in the international trade of animals and animal products.

Rift Valley fever (RVF)- commonly seen in domesticated animals in sub-Saharan Africa, such as cattle, buffalo, sheep, goats, and camels. People can get RVF through contact with blood, body fluids, or tissues of infected animals, or through bites from infected mosquitoes

Enzootic: Endemic in animals. An enzootic disease is constantly present in an animal population, but usually only affects a small number of animals at any one time. An example of an enzootic disease would be the influenza virus in some bird populations or, at a lower incidence, the Type IVb strain of VHS in certain Atlantic fish populations.

Viral hemorrhagic septicemia (VHS) is a deadly infectious fish disease caused by Piscine which afflicts over 50 species of freshwater and marine fish in several parts of the Northern Hemisphere.

Enzootic and epizootic are analogous to endemic and epidemic, respectively. Enzootic means something that affects a population of non-human animals in a limited  region whereas epizootic is relatively more widespread.

Types of Infections

 Infection is invasion and multiplication of microorganism in a host, it does not always cause disease. Disease will not occur if the host’s immunity is active.

Parasites are those microorganisms which can establish themselves and multiply in the hosts. Parasites can be pathogens (pathos (G) =suffering; gen=causing) or commensals (com= sharing ; mensa=table; sharing table=living together)

Pathogens are microorganisms that can produce disease in a host. The process of attacking the host and causing disease is known as pathogenesis.

Commensals are microorganisms that live in a state of harmony with the host, without causing any damage. The normal bacterial flora of the body are largely commensals. They maybe facultative pathogens, causing disease if the immunity of the host is lowered.

A disease resulting from an infection is an infectious disease.

Infections can be of many types

Primary Infection - It is the first or initial infection caused by a parasite/microorganism in a host.

Re-infection-Subsequent infections by the same parasite in the host. For example, a malaria can come again/recur in an individual if exposed to it repeatedly.

Secondary Infection - A new microorganism/parasite causes an infection in a host already suffering from a pre-existing infection and has lowered immunity, it is a secondary infection eg; fungal infection following pulmonary tuberculosis in a patient

Focal Infection/Focal sepsis- infection/sepsis at a particular site eg; appendicitis, tonsilitis. Does not spread to other places.

Iatrogenic Infections- Physician induced infections- resulting from different investigative, diagnostic or other procedures in a hospital eg, abscess/deep wound after an injection

Nosocomial Infections- Hospital acquired infections- Health care associated infections (HAI)

Cross Infection - If a patient already suffering from an infection gets another infection from another host or any other external source, it is a cross infection. Infections acquiring from hospitals are due to cross infection. 

Depending on the Source of infection

• Endogenous Infection – If source of infection is from within the host’s own body – eg. Peritonitis- following intestinal injury, if gut flora causes infection

• Exogenous infection- If source of infection is from outside the host’s own body

Depending on the clinical effects of infection,

• Inapparent/Subclinical Infection- where clinical symptoms of infection are not seen

• Atypical Infection- where the characteristic/typical clinical symptoms are not seen

• Latent Infection -where some microorganisms, after the infection, remain hidden in the tissues in a latent/dormant form. They produce disease when the host immunity is lowered/compromised, later.