The flu is caused by influenza viruses, but many distinct viruses (most commonly rhinoviruses) can cause a cold.Common cold symptoms typically develop about one to three days after exposure to cold-causing viruses. These viruses can be spread through the air, personal contact, and respiratory secretions — encounters such as a handshake, touching contaminated objects, and exposure to an infected person’s sneezes or coughs, notes the Mayo Clinic.Shouting, singing, or even simply talking can also release contaminated droplets into the air, which can then be inhaled, transmitting the virus, according to Houston Methodist.Certain groups of people are more susceptible to complications from getting a cold or the flu, including the very young, older adults, and people with a compromised immune system.Factors that can increase your risk of becoming infected include:AgeChildren younger than 5 and adults over 65 are at a higher risk for developing complications from the flu, according to the CDC.SeasonSeasonal flu activity typically occurs between October and May (flu season), although flu viruses are around all year, per the CDC.Similarly, most people develop colds in the winter and spring, but they can occur anytime, notes MedlinePlus.Weakened Immune SystemViruses can more easily infiltrate the body if you have a weakened immune system. Certain chronic illnesses, such as cancer, HIV or AIDS, and autoimmune diseases, raise your risk of catching a cold or the flu, according to the CDC. Using immunosuppressive medications, such as steroids, biologics, chemotherapy, or transplant medications, also raises your risk.SmokingChronic smoking makes your respiratory system more vulnerable to cold and flu viruses and complications. The American Lung Association reports that cold symptoms tend to be more severe in smokers.PregnancyWomen in their second or third trimester are particularly susceptible to complications from the flu. “We’re not exactly sure why, but there has always been a question of whether or not the immune system changes during pregnancy,” says Laura Riley, MD, the chair of obstetrics and gynecology at Weill Cornell Medicine and the obstetrician-gynecologist in chief at NewYork-Presbyterian Hospital in New York City.RELATED: 8 Ways to Keep Your Immune System HealthyWhat Types of Flu Are There?There are four types of influenza viruses: A and B, which are most commonly associated with seasonal flu activity and epidemics; C, which is relatively rare and causes mild respiratory illness; and D, which primarily affects cattle, according to the CDC.Influenza AThere are many subtypes of influenza A viruses, based on two proteins — hemagglutinin (H) and neuraminidase (N) — found on the surface of the viruses. Two strains of influenza A found in human beings are the H1N1 strain and the H3N2 strain, according to the CDC.A novel strain of H1N1 known as swine flu, because it’s typically spread among pigs, led to a flu pandemic in 2009. Between 2009 and 2010, some estimate that at least 20 percent of the world’s population was infected with swine flu, resulting in about 200,000 deaths, according to Reuters. The influenza pandemic of 1918 was an H1N1 virus of avian origin, according to the Cleveland Clinic.H3N2 mutates more rapidly than other strains, which can make it particularly resistant to the flu vaccine, notes the CDC.Influenza BLess common than influenza A, these viruses cause similar symptoms and can lead to seasonal outbreaks. Per the CDC, influenza B is not categorized by subtypes, but there are two lineages of the virus: Yamagata and Victoria.Influenza CLike influenza A and B, these viruses are found in humans. But influenza C viruses are milder and not thought to cause epidemics. Seasonal flu vaccines, which contain strains of influenza A and B, do not protect against influenza C viruses, cautions the CDC.Influenza DThis strain of influenza is not known to cause illness in humans. A relatively new strain, it primarily affects cattle, though one report notes that it could eventually pose more of a threat to humans.