Infectious Diseases - Session 1

Infectious diseases are illnesses caused by microorganisms such as bacteria, viruses, fungi, or parasites. These diseases can be spread directly or indirectly. They come in various forms. They range from common infections like the flu and the common cold to more severe illnesses.

Some infectious diseases are acute. They develop rapidly and have a short duration. Some diseases can become chronic conditions or result in long term complications or death if not treated.Infectious diseases are mostly caused by pathogens. Can you tell what are pathogens?.

Pathogens are microscopic organisms that can cause disease in humans, animals and plants. These microscopic organisms can be bacteria, viruses, fungi and parasites.They are the agents responsible for the vast majority of infectious diseases. Understanding pathogens is crucial for preventing and treating infectious illnesses.

Here is a breakdown of the different types of pathogens. Bacteria are single-celled microorganisms that can be found everywhere on Earth. While many bacteria are harmless and some are beneficial, some species are pathogenic. Pathogenic bacteria can cause diseases such as strep throat, tuberculosis, cholera and urinary tract infections. Bacterial infections are often treated with antibiotics.

Viruses are tiny infectious agents that cannot replicate on their own and rely on host cells to reproduce. Viruses are incredibly small, typically ranging in size from about twenty nanometers to three hundred nanometers. They are much smaller than bacteria and cannot be seen with a standard light microscope. They are only visible using an electron microscope. They consist of genetic material enclosed in a protein coat.

Genetic material can be either DNA or RNA, which carries the instructions for making new virus particles. A protein coat surrounds and protects the viral genetic material. This protein coat is called the capsid. The capsid is made up of repeating protein subunits called capsomeres. Some viruses have an additional outer lipid envelope derived from the host cell membrane. This envelope might contain viral proteins and glycoproteins.

The virus attaches to specific receptors on the surface of the host cell. The virus injects its genetic material into the host cell or is taken up by the cell through endocytosis.The viral genetic material takes over the host cell's machinery to replicate and transcribe viral genes, producing viral proteins. New viral particles are assembled from the replicated genetic material and viral proteins. Finally, newly formed viruses are released from the host cell. They often destroy the host cell during this process.

Parasites are the pathogens that live in or on another organism and benefit at the expense of the host. Parasitism is a form of symbiotic relationship where the parasite benefits, while the host is harmed. Protozoa are single celled organisms that can parasitize humans and animals. Examples include Plasmodium, Giardia, Toxoplasma and Trypanosoma.

Malaria is a life threatening disease. Malaria is caused by parasites of the genus Plasmodium. Plasmodium falciparum is the most deadly species responsible for the majority of malaria related deaths worldwide. Other species that can cause malaria in humans include Plasmodium vivax, Plasmodium ovale, Plasmodium malariae and more rarely Plasmodium knowlesi. Malaria is primarily transmitted through the bite of infected female Anopheles mosquitoes.

The transmission cycle of malaria begins when a female Anopheles mosquito, infected with malaria parasites, takes a blood meal from a human host. During this blood meal, the mosquito injects saliva into the human's skin to facilitate blood feeding. If the mosquito is carrying malaria parasites, it also inadvertently injects sporozoites into the human's bloodstream. Sporozoites are the infective stage of the parasite.

Once in the bloodstream, the malaria sporozoites quickly travel to the liver, where they invade liver cells. Inside the liver cells, the sporozoites undergo a period of rapid multiplication, eventually developing into thousands of merozoites.After a few days, the infected liver cells rupture, releasing the merozoites into the bloodstream. This marks the beginning of the erythrocytic stage of the malaria infection.

The merozoites invade the erythrocytes and begin to replicate asexually. This replication cycle results in the destruction of the infected red blood cells, releasing more merozoites into the bloodstream. The released merozoites can infect other red blood cells, perpetuating the cycle of infection.As the malaria parasites replicate and destroy red blood cells, the infected individual begins to experience clinical symptoms. These symptoms are fever, chills, sweats, fatigue and muscle aches.

Meanwhile, another part of the transmission cycle is occurring in the mosquito population. An uninfected female Anopheles mosquito takes a blood meal from an infected human. It ingests both blood and malaria parasites in the process. Once inside the mosquito's midgut, the male and female gametocytes of the malaria parasite undergo sexual reproduction, forming zygotes. The zygotes develop into motile forms called ookinetes. Ookinetes traverse the mosquito midgut wall and develop into oocysts on the outer surface of the midgut.

Over time, the oocysts mature and release thousands of sporozoites into the body cavity of mosquito. These sporozoites migrate to the mosquito's salivary glands. In salivary glands, they become concentrated and are ready to be injected into a new human host during a subsequent blood meal. When the infected mosquito takes another blood meal from a human host, the cycle of malaria transmission repeats, perpetuating the spread of the disease.

The treatment of malaria depends on factors such as the species of the parasite and the severity of the infection. Artemisinin based combination therapies are currently the most effective treatment for uncomplicated falciparum malaria. The other antimalarial medications, such as chloroquine or primaquine can be used for other species or in specific situations. Severe malaria requires hospitalization and intravenous administration of antimalarial medications.