Malaria is caused by a protozoan parasite in the genus Plasmodium, which are protists—eukaryotic microorganisms that are not bacteria, fungi, or viruses. These parasites are transmitted to humans by Anopheles mosquitoes. After entering the body, Plasmodium develops in the liver and then invades red blood cells, producing the fever cycles and anemia characteristic of the disease. Understanding this as a protist infection helps explain its life cycle and why treatments target the parasite itself rather than bacterial or viral mechanisms.

Non-communicable diseases are not transmitted from person to person. They develop from a mix of genetic factors, age, lifestyle choices, and environmental influences, rather than from infectious agents that spread between people. This means they aren’t contagious through usual routes like touch, air, or shared objects. Examples include heart disease, type 2 diabetes, cancers, and chronic respiratory diseases. The statement that NCDs are not transmitted captures their defining feature, unlike infectious diseases which can spread between individuals. Saying they are always contagious is incorrect, and claiming they are caused only by viruses is also incorrect, since NCDs arise from non-infectious factors.

Diseases in animals and plants can spread through several routes, not just one. The best answer recognizes three major pathways: direct contact, water, and air. Direct contact allows pathogens to move when a healthy host touches an infected one or comes into contact with contaminated secretions or tissues. Water serves as a vehicle when pathogens are carried in drinking water or through water splash and runoff that reach other hosts. Airborne transmission includes droplets or aerosols produced by an infected organism that can be inhaled or settle on surfaces, enabling spread even without close contact. Because all three routes can contribute to spread, this option best captures how diseases move between individuals and plants in real-world settings. The other options are too limited, describing only one route and missing the others.

The essential idea is the form of the agent used in the vaccine. Living vaccines use attenuated (weakened) bacteria or viruses that can still replicate in the vaccinated person, though they are not capable of causing disease. Because they can replicate, they present the immune system with a broad set of antigens in a way that closely resembles natural infection, which often leads to a strong and long-lasting immune response with memory. They can even induce mucosal immunity when given through certain routes, but they carry a small risk to people with weakened immune systems and, rarely, can revert to a harmful form; they also tend to require careful storage. Killed vaccines use pathogens that have been inactivated and cannot replicate. They still provoke an immune response, but typically not as robust or durable as live vaccines, so they usually require additional doses and sometimes adjuvants to boost protection. They are generally safer for immunocompromised individuals and do not carry a risk of reversion to virulence, though they don’t stimulate as strong a cellular response as living vaccines.