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Disease Transmission
| Question | Answer |
|---|---|
| Modes of Disease Transmission: | Pathogens can be classified by their mechanism of transmission. Pathogen transmission can be direct or indirect. |
| Direct Host to Host Transmission: | transmission from one host to another. Examples include STIs, common cold, flu, and ringworm. |
| Indirect Host to Host: | transmission via vectors or vehicles such as fomites, water, and food. |
| Indirect Common Source: | transmission via vehicles that serve as a common source of infection for multiple individuals. |
| Common-Source Epidemic (e.g., Cholera): | Caused by exposure to a single contaminated source (like water or food). Characterized by a sharp rise and fall in cases (short-lived outbreak). Example shown: red curve with rapid onset and decline in disease incidence. |
| Host-to-Host Epidemic (e.g., Influenza): | Disease spreads person-to-person. Gradual rise and slower decline in cases as transmission continues between hosts. Example shown: green curve with slower onset and longer duration. |
| Example of Epidemic Curves: | The Zika virus outbreak shows gradual increase and decline, consistent with host-to-host spread. Foodborne outbreak shows a sharp, single-day spike in cases—typical of a common-source epidemic. |
| Who Harbors the Pathogens? | Disease carriers and disease reservoirs. |
| Reservoirs: | sites in which infectious agents remain viable and from which individuals can become infected. Examples include humans, animals, non-living water (soil, water). |
| Carriers: | Individuals (often humans) who harbor a pathogen without showing symptoms but can transmit it to others. Example: A person with asymptomatic typhoid fever. |
| Vectors: | Living organisms — usually arthropods like mosquitoes or ticks—that transmit pathogens from one host to another. Example: Anopheles mosquitoes transmit malaria. |
| Vehicles: | Nonliving (inanimate) materials or objects that carry infectious agents to a new host. Example: Contaminated water, food, or blood transfusions. |
| Zoonoses: | Diseases that are naturally transmitted between animals and humans. Example: Rabies, plague, or COVID-19. |
| Zoonosis Control: | Managing a zoonotic disease in humans may not eliminate it as a public health risk, since some pathogens require transfer between nonhuman hosts and humans to complete their life cycle. |
| Mary Mallon (Typhoid Mary): | An Irish-born American cook who infected 51–122 people with typhoid fever. She was quarantined twice, the second time for life, because she continued working as a cook and exposed others to the disease. |
| Toxoplasmosis: | Caused by the protozoan Toxoplasma gondii, often spread via undercooked meat or cat feces (cats are vectors). It affects the nervous system and eyes and can be dangerous for unborn babies if a pregnant woman is infected. |
| Chagas Disease: | Caused by the protozoan Trypanosoma cruzi and transmitted by the triatomine (kissing) bug, Chagas disease primarily affects the heart and digestive system. |
| Lyme Disease: | Caused by Borrelia burgdorferi and transmitted by black-legged ticks (Ixodes). Affects the skin, joints, nervous system, and sometimes heart, often starting with a bull’s-eye rash and flu-like symptoms. |
| Malaria: | Caused by Plasmodium parasites and transmitted by Anopheles mosquitoes, malaria infects the blood and liver. It leads to cycles of fever, chills, and anemia due to the destruction of red blood cells. |
| Rabies: | Rabies is a viral disease caused by the Rabies virus (Lyssavirus) and transmitted through the bite or saliva of infected animals, commonly dogs or bats. It attacks the nervous system, leading to paralysis, confusion, and death once symptoms appear. |
| Zika Virus Disease: | Caused by the Zika virus (Flavivirus) and spread by Aedes mosquitoes, this infection usually causes mild fever, rash, and joint pain. It mainly affects the nervous and reproductive systems, and can cause birth defects. |
| Signs Vs. Symptoms: | Signs are objective — things that can be seen, measured, or observed by others (like a doctor). Symptoms are subjective — things that the patient feels or reports, which cannot be directly measured by others. |
| Examples of Signs: | Fever, rash, high blood pressure, swelling, vomiting. |
| Examples of Symptoms: | Pain, fatigue, dizziness, nausea, headache. |
| Syndrome: | set of signs and symptoms that occur together and characterize a particular disease or condition. |
| Incubation Period: | the time between first exposure to a pathogen and the appearance of initial symptoms; the pathogen multiplies silently, and this stage can last from a few hours (like with food poisoning) to several years (like with HIV). |
| Prodromal Period: | Early phase with mild symptoms like fatigue or fever, lasting hours to a few days. Illness period: Symptoms peak in severity, lasting days to weeks depending on the infection (most contagious). |
| Decline Period: | occurs when the immune system (or treatment) overcomes the pathogen, and symptoms begin to lessen; this stage typically lasts several days. |
| Convalescence Period: | is the recovery phase, where the body repairs tissue and regains strength, lasting from days to months depending on how serious the disease was. |
| Which is the longest stage of disease? | The incubation period is usually the longest stage of a disease. It can last weeks, months, or even years, while the other stages (prodromal, illness, decline, and convalescence) usually last days to weeks. |
| Streptococcal Disease: | Streptococcus pyogenes is the major species in the group A streptococci, commonly found in low numbers in the upper respiratory tract of healthy individuals. Can cause pus-forming wounds. |
| Streptococcus Pyrogens | Causes strep throat and can infect the ear, mammary glands, and skin. Skin infections include impetigo (pus-filled blisters) and erysipelas. Occurs when host defenses are weak or a virulent strain is introduced. |
| Erysipelas: | It is most often caused by Streptococcus pyogenes (Group A Streptococcus). The infection usually appears as a bright red, raised, and sharply bordered rash, often on the face or legs. |
| Pertussis (whooping cough): | Highly contagious respiratory disease caused by Bordetella pertussis, often in school-age children. Characterized by recurrent, violent coughing. Unimmunized individuals are at high risk of infection and transmission. |
| Tuberculosis: | M. Tuberculosis is transmitted by airborne droplets. Classified as a primary infection (initial infection) or postprimary infection (reinfection). |
| Primary Tuberculosis Infection: | Occurs when Mycobacterium tuberculosis first enters the body, making the immune system hypersensitive to the bacteria. Detected by a tuberculin skin test, showing redness and swelling if prior exposure exists. |
| Secondary Tuberculosis Infection: | chronic tuberculosis often results in gradual spread of tubercular lesions in the lungs. |
| Airborne Bacterial Diseases: | streptococcal diseases, pertussis, and tuberculosis are all airborne bacterial diseases. |
| Measles: | often affects susceptible children as an acute, highly infectious, often epidemic disease. Caused by a paramyxovirus. |
| Paramyxovirus: | a single-stranded, minus-sense RNA virus. Viruses enter the nose and throat by airborne transmission. |
| Measles Prevention: | Vaccination with the MMR (measles, mumps, rubella) vaccine greatly reduces cases. |
| Measles Epidemiology: | Cases dramatically declined after the introduction of the measles vaccine in 1963 (inactivated) and 1968 (live). |
| MMR: | Stands for Measles, Mumps, and Rubella, a combined vaccine that protects against three highly contagious viral diseases. |
| Varicella-Zoster Virus (VZV): | Common childhood disease characterized by a systemic papular rash. Caused by varicella-zoster virus (VZV), a herpesvirus. VZV is highly contagious and transmitted by infectious droplets. A vaccine is currently in the United States. |
| Rubella: | caused by a positive-strand RNA virus of the togavirus group. Disease symptoms resemble measles but are generally milder and less contagious. Routine childhood immunization is practiced in the United States. |
| Mumps: | A highly contagious disease caused by a paramyxovirus, marked by painful swelling of the salivary glands. It mainly affects young adults (18–34) and has caused periodic outbreaks. Immunization with the MMR vaccine helps prevent infection. |
| Primary Infection (Varicella/Chickenpox): | VZV enters via the respiratory tract or conjunctiva, spreads through the bloodstream, causing a generalized vesicular rash, and then travels along sensory nerves to establish infection in the dorsal root ganglia. |
| Latency Period: | After primary infection, VZV remains dormant within sensory neurons of the dorsal root ganglia for the person's lifetime. During this phase, the virus produces no symptoms and is controlled but not eliminated by the immune system. |
| Reactivation (Herpes Zoster/Shingles): | Latent VZV reactivates when immunity declines (aging, stress, immunosuppression) and travels down sensory nerves to the skin, causing a painful, unilateral vesicular rash, often in the elderly or immunocompromised. |
| Shingles (another VZV infection): | Mainly affects the elderly and immunocompromised. Prevention: Zostavax (live attenuated, 50+) and Shingrix (recombinant, ~90% effective, 2 doses) reduce the risk of viral reactivation and shingles. |
| Influenza: | influenza viruses contain a single-stranded, negative-sense, segmented RNA genome surrounded by an envelope composed of protein, a lipid bilayer, and external glycoproteins. |
| How many types of influenza viruses are there? | There are three types. Influenza A is the most important human pathogen. |
| Influenza A Virus Identification: | Each strain is identified by unique surface glycoproteins on its viral envelope. |
| Hemagglutinin (HA or “H antigen”): | A surface protein that helps the influenza virus attach to host cells. |
| Neuraminidase (NA or “N antigen”): | A surface protein that helps the influenza virus release from host cells after replication. |
| Virus Naming (Example: H1N1): | Each influenza virus has one HA and one NA type, and the strain name reflects these (e.g., H1N1). |
| HA and NA Composition: | Each antigen is made of several individual proteins that determine influenza viral behavior and immune recognition. |
| Antigenic Drift (Influenza): | A gradual, minor change in the influenza virus that occurs through small gene mutations in the viral RNA. Antigenic drift happens continuously and is responsible for seasonal flu outbreaks and the need for annual flu vaccines. |
| Antigenic Shift (Influenza): | Major change in influenza antigens due to gene reassortment. Occurs when avian and human influenza viruses infect pigs simultaneously, mixing RNA segments to create a new virus that can infect humans and potentially cause a pandemic. |
| Negative ssRNA viruses (Flu): | Virus carries its own RNA-dependent RNA polymerase (replicase) because its RNA cannot be directly read by host ribosomes. |
| Positive ssRNA viruses (SARS): | Viral RNA can be directly translated by host ribosomes to produce viral proteins. |
| Herd Immunity: | When a large portion of a population is immune to an infection, reducing its spread and protecting those who are not immune. |
| Koplik Spots: | Small, white spots with a red halo inside the mouth, characteristic of early measles infection. |
| Chicken-Pox Parties: | Social gatherings where children are intentionally exposed to varicella (chickenpox) to gain immunity early in life. |
| Airborne Viral Diseases: | MMR, Varicella-Zoster, Influenza. |
| Sexually Transmitted Diseases: | Gonorrhea & Syphilis are the diseases of note. Sexually transmitted diseases (STDs) are infections spread primarily through sexual contact, caused by bacteria, viruses, or parasites. |
| Gonorrhea & Syphilis: | preventable, treatable bacterial STIs. The overall pattern of diseases differs between the two → gonorrhea is prevalent and often asymptomatic in women. Syphilis has a low prevalence and exhibits very obvious symptoms. |
| Gonorrhea: | Caused by Neisseria gonorrhoeae. In females, often mild or unnoticed vaginitis, which can lead to pelvic inflammatory disease if untreated. In males, causes a painful urethral infection. |
| Syphilis: | Syphilis is a sexually transmitted infection caused by Treponema pallidum that progresses in stages, from painless sores to systemic and potentially severe late complications. |
| Why are STIs rising in older populations? | Rising because many remain sexually active, often with new partners, but use condoms less and have lower STI awareness. Age-related changes, like thinner vaginal tissue and a weaker immune system, also increase susceptibility. |
| Legionella Pneumophilia: | A water/food-borne disease. Gram-negative bacterium that causes legionellosis and thrives in residential water system biofilms where it's protected from chlorine. These biofilms serve as reservoirs for waterborne transmission of the disease. |
| Legionellosis: | a severe form of pneumonia caused by the bacterium Legionella, typically spread through inhaling contaminated water droplets. It can cause fever, cough, muscle aches, and sometimes life-threatening respiratory complications. |
| Where is Legionella Pneumophilia Found? | Found in improperly sanitized water systems like cooling towers, air conditioners, hot water tanks, spas, and poorly chlorinated pools. Spreads via aerosolized water droplets, not person-to-person. Cases rose significantly from 2000–2018. |
| Typhoid Bacterium: | Salmonella enterica (typhi) is a Gram-negative, peritrichously flagellated bacterium causing typhoid fever. It is a major waterborne pathogen, transmitted via fecally contaminated water. |
| Norovirus (Norwalk Agent): | Causes vomiting, diarrhea, and malaise by infecting intestinal tuft cells, rarely fatal. Spreads fecal-orally, often via contaminated water or outbreaks in group settings like cruise ships and care facilities. |
| Vomiting Larry: | training model used to demonstrate how far vomit—and the norovirus within it—can spread. |
| Food Poisoning Time Frame (Infection and Recovery Time): | Infection (onset): Symptoms usually appear 12–48 hours after exposure. Recovery: Most people recover within 1–3 days, though severe cases can last longer. |
| Food poisoning (intoxication): | results from ingesting preformed microbial toxins in food, where the microorganisms don't need to grow in the host. |
| Food infection: | occurs when pathogens in contaminated food are ingested and subsequently grow within the host to cause illness. |
Created by:
smurtab