Hello, and welcome to today's biology lesson. Today, we are diving into a crucial topic: Diseases, their Causes and Control. By the end of this session, you will understand what disease really means, how diseases are classified, and how bacteria, viruses, protozoa, and worms cause some of the most significant illnesses affecting humanity. We will also explore how to prevent and control these conditions.
Let us begin with a fundamental question: what exactly is a disease?
A disease is defined as a condition in which the normal functioning of the body is disturbed. More precisely, it is a departure from normal health through structural or functional disorder of the body. Some diseases are mild and resolve quickly, while others can become serious and even fatal.
Human understanding of disease has evolved dramatically. Early humans believed diseases were punishments from gods or acts of evil spirits, and they relied on magic and sorcery for cures. Today, science recognizes that every disease has a definite cause, which means we can prevent or cure it. To promote global health awareness, we celebrate World Health Day every year on April seventh.
Now, let us explore how we categorize diseases.
First, diseases can be classified by their extent of occurrence. An endemic disease is restricted to a particular area and affects relatively few people. Yellow fever in certain African countries and goitre in sub-Himalayan regions are examples.
An epidemic disease breaks out and spreads rapidly from place to place, affecting large numbers of people simultaneously. The plague outbreak in India in nineteen ninety-four, spreading from Surat to multiple cities, was a classic epidemic.
A pandemic disease is distributed worldwide. AIDS is a devastating example you are already familiar with.
Finally, sporadic diseases appear as scattered individual cases. Malaria and cholera sometimes appear in this pattern.
The second major way to classify diseases is by communicability, meaning whether they can spread from person to person.
Non-communicable or non-infectious diseases have no germs involved and cannot spread from one person to another. Diabetes, colour blindness, heart attacks, and beri-beri fall into this category.
Communicable or infectious diseases, on the other hand, are caused by germs called pathogens. These pathogens travel from an infected person to a healthy one, a process we call infection. Importantly, symptoms do not appear immediately. There is a delay called the incubation period, which can range from a few hours to many days, or even years in some cases. Cholera, smallpox, and malaria are common examples of infectious diseases.
Let us now examine the four main types of pathogens and the diseases they cause.
First, bacteria. Cholera attacks the intestinal tract, causing severe vomiting and diarrhoea. The patient becomes dangerously dehydrated, and without prompt treatment with saline water, death can occur. The bacterium spreads through contaminated food and water, often carried by flies. Prevention requires good sanitation, covering food, boiling drinking water, and vaccination.
Typhoid fever presents with continuous fever that typically rises in the afternoon, along with reddish eruptions on the chest and abdomen. The bacterium attacks the intestines and spreads through contaminated food, milk, and flies. The antibiotic chloromycetin is effective, and annual vaccination helps prevent infection.
Tuberculosis, or TB, primarily affects the lungs but can spread to the brain, kidneys, and bones. It spreads through sputum and dust in the air. Remarkably, most people encounter these germs and overcome minor infections, but when body resistance is low, the disease becomes severe. Streptomycin is the antibiotic of choice, and BCG vaccination builds immunity. March twenty-fourth is observed as Anti-Tuberculosis Day.
Next, let us turn to diseases caused by protozoa.
Malaria is caused by the protozoan Plasmodium and transmitted by the bite of an infected female Anopheles mosquito. When this mosquito bites an infected person, it sucks up the parasite, which multiplies in the mosquito's gut. The mosquito then becomes a vector, transmitting the disease to healthy people through its bite. The parasite destroys red blood cells, causing recurring chills and high fever every three to four days. Prevention focuses on destroying mosquitoes and using protective measures like nets and repellents.
Amoebic dysentery, or amoebiasis, is caused by Entamoeba histolytica. It destroys the lining of the large intestine, causing diarrhoea with mucus and sometimes blood. Flies contaminate food, spreading the infection. Proper sanitation and protecting food from dust and flies are essential preventive measures.
Sleeping sickness is caused by Trypanosoma protozoa and transmitted by the tsetse fly. Early symptoms include fever, headache, and joint pains. Later, the disease affects the nervous system, causing confusion, poor coordination, and sleep disturbances that can be fatal.
Now we come to diseases caused by parasitic worms, also called helminths.
Ascariasis is caused by the common roundworm, Ascaris lumbricoides. These worms live in the intestines, absorbing digested food from the host. Female worms lay thousands of eggs daily, which pass out with faeces and contaminate soil and vegetables. Children playing on the ground can ingest eggs through unwashed hands or raw vegetables.
Taeniasis is caused by tapeworms, specifically Taenia solium from pork, though a similar species from beef also exists. Picture a worm that can grow to a metre or more in your intestine, absorbing most of your digested food. The patient becomes terribly weak. The worm releases segments containing eggs, which pass out with faeces. When pigs or cows ingest these eggs, larvae form in their muscles. Humans become infected by eating raw or undercooked pork or beef.
Filariasis, also known as elephantiasis, is caused by Wuchereria bancrofti. It is transmitted by the Culex mosquito. The worms lodge in the lymphatic system, blocking lymph circulation. This causes severe swelling of limbs, making them resemble elephant legs. Eradicating mosquitoes and using antibiotics are key control measures.
Finally, let us explore viral diseases.
Viruses are extraordinary entities. They are extremely small, visible only through electron microscopes, and composed of nucleic acids, specifically RNA, and proteins. They cannot live freely in nature, only inside host cells, which they commandeer to produce more viruses. Viruses can be crystallized and stored, yet they multiply like living things inside hosts. This makes them a fascinating connecting link between living and non-living matter.
AIDS, or Acquired Immuno Deficiency Syndrome, is the last stage of HIV infection, caused by the Human Immunodeficiency Virus. This virus attacks the immune system itself, the very cells that defend against infections. Patients become vulnerable to even minor infections and cancers. The incubation period can exceed ten to twelve years, during which infected persons are HIV-positive but may show no symptoms of AIDS. Once AIDS fully develops, death usually occurs within three years.
AIDS spreads through sexual contact, contaminated blood transfusions, mother-to-child transmission during pregnancy or birth, and shared unsterilized needles. Importantly, it does not spread through casual contact, sharing clothes, shaking hands, or using common bathrooms. World AIDS Day is observed on December first.
Chicken pox, caused by the Varicella zoster virus, mainly affects children. It spreads through close contact and produces highly irritating rashes that begin on the chest and back, then spread to limbs and face. The rashes progress from pink spots to watery blisters that dry into scabs. Bed rest, keeping rashes clean and dry, avoiding pricking blisters, and vaccination at 12 to 18 months are recommended.
Hepatitis, meaning inflammation of the liver, is caused by five different hepatitis viruses: A, B, C, D, and E. Hepatitis A spreads through contaminated food and water, while Hepatitis B spreads through contaminated syringes and blood transfusions. Symptoms include fever, headache, loss of appetite, nausea, and eventually jaundice with yellow urine and pale stools. Bed rest and a high-calorie diet with limited protein and fat aid recovery.
COVID-19, caused by SARS-CoV-2, emerged in Wuhan, China in December twenty nineteen and became a devastating pandemic. It spreads through airborne particles and respiratory droplets. Symptoms include fever, cough, sore throat, breathing difficulties, and sometimes loss of smell and taste. Prevention measures include staying home, hand washing, avoiding crowds, wearing masks, maintaining distance, and getting vaccinated. There is no specific cure; treatment focuses on supportive care and oxygen when needed.
Before we conclude, let us quickly recap the essential points.
First, disease is a condition where normal body functioning is disturbed, and every disease has a definite cause.
Second, diseases are categorized by extent of occurrence as endemic, epidemic, pandemic, and sporadic; and by communicability as infectious and non-infectious.
Third, the incubation period is the time between germ entry and symptom appearance, varying from hours to years.
Fourth, bacteria cause cholera, typhoid, and tuberculosis; protozoa cause malaria, amoebic dysentery, and sleeping sickness; worms cause ascariasis, taeniasis, and filariasis; and viruses cause AIDS, chicken pox, and hepatitis.
Fifth, prevention strategies include sanitation, vaccination, vector control, safe food and water practices, and avoiding risky behaviours.
Sixth, AIDS specifically paralyzes the immune system, has no cure yet, and requires understanding of its true transmission routes to combat stigma and prevent spread.
Understanding diseases empowers you to protect yourself and your community. Stay curious, stay informed, and remember that prevention is always better than cure. Thank you for listening, and see you in the next lesson.