Disease is defined as any deviation from the normal structure or function of any body part, organ, or system, manifested by a characteristic set of symptoms. It can be broadly classified into two categories: infectious and non-infectious diseases. Infectious diseases are caused by the invasion of pathogens, which can be transmitted from one host to another. Non-infectious diseases arise from genetic, environmental, or lifestyle factors and are not caused by infectious agents.
Non-Infectious Diseases
These diseases are not caused by pathogens and cannot be spread from person to person. Instead, they result from internal factors such as genetic mutations or external factors like environmental exposures and lifestyle choices. Non-infectious diseases include conditions like diabetes, respiratory diseases, anaemia and scurvy. These diseases are often chronic and require management through lifestyle changes or medication.
Infectious Diseases
These diseases are caused by infectious agents, including prions, viruses, bacteria, fungi, protists and parasites. Infectious diseases occur when these agents invade the host body, reproduce, and disrupt normal bodily functions. They can spread between individuals or through vectors like contaminated food, water, or insects. Examples of infectious diseases include influenza, tuberculosis and malaria.
Bacteria
Bacteria are single-celled prokaryotic organisms that lack a nucleus and membrane-bound organelles. They possess a cell wall that can vary in structure depending on the type of bacteria. Pathogenic bacteria can cause diseases such as strep throat and cholera.
Fungi
Fungi are eukaryotic organisms that digest their food externally by secreting enzymes onto their host and then absorbing the nutrients. They can be pathogenic, causing diseases like athlete’s foot and candidiasis.
Viruses
Viruses are composed of a DNA or RNA genome encased in a protein shell called a capsid. Some viruses also have an external lipid envelope. Viruses cannot reproduce on their own and must invade host cells to replicate. Diseases caused by viruses include HIV/AIDS and the common cold.
Protists
Protists are diverse eukaryotic microorganisms, and while many are non-pathogenic, some, such as certain protozoa, are pathogenic. They cause diseases like malaria and amoebic dysentery.
Parasites
Parasites are eukaryotic organisms that live on or inside a host organism, obtaining nourishment at the host’s expense. They include protozoa, helminths (worms) and ectoparasites such as lice and ticks, causing diseases like giardiasis and schistosomiasis.
Prions
Prions are the smallest of all pathogenic agents and are unique because they do not contain any genetic material. They are abnormal proteins that induce normal proteins in the brain to fold incorrectly. This misfolding creates a chain reaction, leading to neurodegenerative diseases such as Creutzfeldt-Jakob disease.