Viral diseases spread when virus particles pass from an infected host to a new one. The main routes of transmission are:• Droplets and aerosols: coughs, sneezes and breathing release tiny droplets containing virus, e.g. influenza, measles, the common cold, COVID-19.
• Direct contact and fomites: touching an infected person or a contaminated surface, then the mouth, nose or eyes.
• Body fluids: blood, semen or vaginal fluids, through unprotected sex, shared needles or transfusions, e.g. HIV, hepatitis B.
• Faecal-oral route: contaminated food or water, e.g. norovirus, poliovirus.
• Vectors: an organism that carries the virus between hosts, e.g. mosquitoes spreading dengue and Zika.
• Vertical transmission: from mother to baby across the placenta, at birth or in breast milk, e.g. HIV.How fast a disease spreads depends on how easily the virus is transmitted, how long people are infectious, whether infected people show symptoms, population density and how much movement there is between groups.A useful measure is the basic reproduction number, R0: the average number of people one infected person passes the disease to, in a population with no immunity. If R0 is greater than 1, the number of cases grows; if it is less than 1, the outbreak dies out.Worked example. A virus has R0 = 3. Starting from one case, each generation of infection is three times larger: 1 → 3 → 9 → 27 → 81. The fourth generation after the first case contains 34 = 81 new cases.Measles, with R0 of about 12 to 18, is one of the most infectious diseases known.
• Direct contact and fomites: touching an infected person or a contaminated surface, then the mouth, nose or eyes.
• Body fluids: blood, semen or vaginal fluids, through unprotected sex, shared needles or transfusions, e.g. HIV, hepatitis B.
• Faecal-oral route: contaminated food or water, e.g. norovirus, poliovirus.
• Vectors: an organism that carries the virus between hosts, e.g. mosquitoes spreading dengue and Zika.
• Vertical transmission: from mother to baby across the placenta, at birth or in breast milk, e.g. HIV.How fast a disease spreads depends on how easily the virus is transmitted, how long people are infectious, whether infected people show symptoms, population density and how much movement there is between groups.A useful measure is the basic reproduction number, R0: the average number of people one infected person passes the disease to, in a population with no immunity. If R0 is greater than 1, the number of cases grows; if it is less than 1, the outbreak dies out.Worked example. A virus has R0 = 3. Starting from one case, each generation of infection is three times larger: 1 → 3 → 9 → 27 → 81. The fourth generation after the first case contains 34 = 81 new cases.Measles, with R0 of about 12 to 18, is one of the most infectious diseases known.