Parasitism and Disease

A parasite is an organism that lives in or on another organism, the host, and obtains nutrients from it, harming the host. Parasites usually do not kill their host quickly, because they depend on it. Examples include tapeworms in the gut, and fleas, ticks and lice on the skin. Pathogens are microorganisms, such as bacteria, viruses, fungi and protoctists, that cause disease.Effects on the host population. Parasites and pathogens reduce host fitness: infected individuals may be weaker, grow more slowly, produce fewer offspring or be more easily caught by predators. Some diseases kill many individuals. For example, myxomatosis, a viral disease spread by fleas, killed over 99% of rabbits in parts of Britain in the 1950s, and ash dieback, caused by a fungus, is killing many ash trees across the UK.Effect of crowding. Parasites and pathogens spread more easily when hosts are crowded, because contact between individuals is more frequent. So their effect is greater in large, dense populations, and they help to keep population size in check.Coevolution. Hosts are under selection pressure to resist infection, and parasites are under pressure to overcome host defences. After the myxomatosis outbreak, rabbits with genetic resistance survived and bred, and less deadly strains of the virus were favoured because they did not kill the host before spreading. Rabbit numbers recovered.Worked example. A population of 2000 rabbits is hit by a disease that kills 85%. Number surviving = 2000 × 0.15 = 300.